Half-assed file truncation

[akaros.git] / kern / src / vfs.c

/* Copyright (c) 2009, 2010 The Regents of the University of California
 * Barret Rhoden <brho@cs.berkeley.edu>
 * See LICENSE for details.
 *
 * Default implementations and global values for the VFS. */

#include <vfs.h> // keep this first
#include <sys/queue.h>
#include <assert.h>
#include <stdio.h>
#include <atomic.h>
#include <slab.h>
#include <kmalloc.h>
#include <kfs.h>
#include <ext2fs.h>
#include <pmap.h>
#include <umem.h>
#include <smp.h>

struct sb_tailq super_blocks = TAILQ_HEAD_INITIALIZER(super_blocks);
spinlock_t super_blocks_lock = SPINLOCK_INITIALIZER;
struct fs_type_tailq file_systems = TAILQ_HEAD_INITIALIZER(file_systems);
struct namespace default_ns;

struct kmem_cache *dentry_kcache; // not to be confused with the dcache
struct kmem_cache *inode_kcache;
struct kmem_cache *file_kcache;

/* Mounts fs from dev_name at mnt_pt in namespace ns.  There could be no mnt_pt,
 * such as with the root of (the default) namespace.  Not sure how it would work
 * with multiple namespaces on the same FS yet.  Note if you mount the same FS
 * multiple times, you only have one FS still (and one SB).  If we ever support
 * that... */
struct vfsmount *__mount_fs(struct fs_type *fs, char *dev_name,
                            struct dentry *mnt_pt, int flags,
                            struct namespace *ns)
{
        struct super_block *sb;
        struct vfsmount *vmnt = kmalloc(sizeof(struct vfsmount), 0);

        /* this first ref is stored in the NS tailq below */
        kref_init(&vmnt->mnt_kref, fake_release, 1);
        /* Build the vfsmount, if there is no mnt_pt, mnt is the root vfsmount (for
         * now).  fields related to the actual FS, like the sb and the mnt_root are
         * set in the fs-specific get_sb() call. */
        if (!mnt_pt) {
                vmnt->mnt_parent = NULL;
                vmnt->mnt_mountpoint = NULL;
        } else { /* common case, but won't be tested til we try to mount another FS */
                mnt_pt->d_mount_point = TRUE;
                mnt_pt->d_mounted_fs = vmnt;
                kref_get(&vmnt->mnt_kref, 1); /* held by mnt_pt */
                vmnt->mnt_parent = mnt_pt->d_sb->s_mount;
                vmnt->mnt_mountpoint = mnt_pt;
        }
        TAILQ_INIT(&vmnt->mnt_child_mounts);
        vmnt->mnt_flags = flags;
        vmnt->mnt_devname = dev_name;
        vmnt->mnt_namespace = ns;
        kref_get(&ns->kref, 1); /* held by vmnt */

        /* Read in / create the SB */
        sb = fs->get_sb(fs, flags, dev_name, vmnt);
        if (!sb)
                panic("You're FS sucks");

        /* TODO: consider moving this into get_sb or something, in case the SB
         * already exists (mounting again) (if we support that) */
        spin_lock(&super_blocks_lock);
        TAILQ_INSERT_TAIL(&super_blocks, sb, s_list); /* storing a ref here... */
        spin_unlock(&super_blocks_lock);

        /* Update holding NS */
        spin_lock(&ns->lock);
        TAILQ_INSERT_TAIL(&ns->vfsmounts, vmnt, mnt_list);
        spin_unlock(&ns->lock);
        /* note to self: so, right after this point, the NS points to the root FS
         * mount (we return the mnt, which gets assigned), the root mnt has a dentry
         * for /, backed by an inode, with a SB prepped and in memory. */
        return vmnt;
}

void vfs_init(void)
{
        struct fs_type *fs;

        dentry_kcache = kmem_cache_create("dentry", sizeof(struct dentry),
                                          __alignof__(struct dentry), 0, 0, 0);
        inode_kcache = kmem_cache_create("inode", sizeof(struct inode),
                                         __alignof__(struct inode), 0, 0, 0);
        file_kcache = kmem_cache_create("file", sizeof(struct file),
                                        __alignof__(struct file), 0, 0, 0);
        /* default NS never dies, +1 to exist */
        kref_init(&default_ns.kref, fake_release, 1);
        spinlock_init(&default_ns.lock);
        default_ns.root = NULL;
        TAILQ_INIT(&default_ns.vfsmounts);

        /* build list of all FS's in the system.  put yours here.  if this is ever
         * done on the fly, we'll need to lock. */
        TAILQ_INSERT_TAIL(&file_systems, &kfs_fs_type, list);
#ifdef CONFIG_EXT2FS
        TAILQ_INSERT_TAIL(&file_systems, &ext2_fs_type, list);
#endif
        TAILQ_FOREACH(fs, &file_systems, list)
                printk("Supports the %s Filesystem\n", fs->name);

        /* mounting KFS at the root (/), pending root= parameters */
        // TODO: linux creates a temp root_fs, then mounts the real root onto that
        default_ns.root = __mount_fs(&kfs_fs_type, "RAM", NULL, 0, &default_ns);

        printk("vfs_init() completed\n");
}

/* Builds / populates the qstr of a dentry based on its d_iname.  If there is an
 * l_name, (long), it will use that instead of the inline name.  This will
 * probably change a bit. */
void qstr_builder(struct dentry *dentry, char *l_name)
{
        dentry->d_name.name = l_name ? l_name : dentry->d_iname;
        // TODO: pending what we actually do in d_hash
        //dentry->d_name.hash = dentry->d_op->d_hash(dentry, &dentry->d_name); 
        dentry->d_name.hash = 0xcafebabe;
        dentry->d_name.len = strnlen(dentry->d_name.name, MAX_FILENAME_SZ);
}

/* Useful little helper - return the string ptr for a given file */
char *file_name(struct file *file)
{
        return file->f_dentry->d_name.name;
}

/* Some issues with this, coupled closely to fs_lookup.
 *
 * Note the use of __dentry_free, instead of kref_put.  In those cases, we don't
 * want to treat it like a kref and we have the only reference to it, so it is
 * okay to do this.  It makes dentry_release() easier too. */
static struct dentry *do_lookup(struct dentry *parent, char *name)
{
        struct dentry *result, *query;
        query = get_dentry(parent->d_sb, parent, name);
        if (!query) {
                warn("OOM in do_lookup(), probably wasn't expected\n");
                return 0;
        }
        result = dcache_get(parent->d_sb, query); 
        if (result) {
                __dentry_free(query);
                return result;
        }
        /* No result, check for negative */
        if (query->d_flags & DENTRY_NEGATIVE) {
                __dentry_free(query);
                return 0;
        }
        /* not in the dcache at all, need to consult the FS */
        result = parent->d_inode->i_op->lookup(parent->d_inode, query, 0);
        if (!result) {
                /* Note the USED flag will get turned off when this gets added to the
                 * LRU in dentry_release().  There's a slight race here that we'll panic
                 * on, but I want to catch it (in dcache_put()) for now. */
                query->d_flags |= DENTRY_NEGATIVE;
                dcache_put(parent->d_sb, query);
                kref_put(&query->d_kref);
                return 0;
        }
        dcache_put(parent->d_sb, result);
        /* This is because KFS doesn't return the same dentry, but ext2 does.  this
         * is ugly and needs to be fixed. (TODO) */
        if (result != query)
                __dentry_free(query);

        /* TODO: if the following are done by us, how do we know the i_ino?
         * also need to handle inodes that are already read in!  For now, we're
         * going to have the FS handle it in it's lookup() method: 
         * - get a new inode
         * - read in the inode
         * - put in the inode cache */
        return result;
}

/* Update ND such that it represents having followed dentry.  IAW the nd
 * refcnting rules, we need to decref any references that were in there before
 * they get clobbered. */
static int next_link(struct dentry *dentry, struct nameidata *nd)
{
        assert(nd->dentry && nd->mnt);
        /* update the dentry */
        kref_get(&dentry->d_kref, 1);
        kref_put(&nd->dentry->d_kref);
        nd->dentry = dentry;
        /* update the mount, if we need to */
        if (dentry->d_sb->s_mount != nd->mnt) {
                kref_get(&dentry->d_sb->s_mount->mnt_kref, 1);
                kref_put(&nd->mnt->mnt_kref);
                nd->mnt = dentry->d_sb->s_mount;
        }
        return 0;
}

/* Walk up one directory, being careful of mountpoints, namespaces, and the top
 * of the FS */
static int climb_up(struct nameidata *nd)
{
        printd("CLIMB_UP, from %s\n", nd->dentry->d_name.name);
        /* Top of the world, just return.  Should also check for being at the top of
         * the current process's namespace (TODO) */
        if (!nd->dentry->d_parent || (nd->dentry->d_parent == nd->dentry))
                return -1;
        /* Check if we are at the top of a mount, if so, we need to follow
         * backwards, and then climb_up from that one.  We might need to climb
         * multiple times if we mount multiple FSs at the same spot (highly
         * unlikely).  This is completely untested.  Might recurse instead. */
        while (nd->mnt->mnt_root == nd->dentry) {
                if (!nd->mnt->mnt_parent) {
                        warn("Might have expected a parent vfsmount (dentry had a parent)");
                        return -1;
                }
                next_link(nd->mnt->mnt_mountpoint, nd);
        }
        /* Backwards walk (no mounts or any other issues now). */
        next_link(nd->dentry->d_parent, nd);
        printd("CLIMB_UP, to   %s\n", nd->dentry->d_name.name);
        return 0;
}

/* nd->dentry might be on a mount point, so we need to move on to the child
 * mount's root. */
static int follow_mount(struct nameidata *nd)
{
        if (!nd->dentry->d_mount_point)
                return 0;
        next_link(nd->dentry->d_mounted_fs->mnt_root, nd);
        return 0;
}

static int link_path_walk(char *path, struct nameidata *nd);

/* When nd->dentry is for a symlink, this will recurse and follow that symlink,
 * so that nd contains the results of following the symlink (dentry and mnt).
 * Returns when it isn't a symlink, 1 on following a link, and < 0 on error. */
static int follow_symlink(struct nameidata *nd)
{
        int retval;
        char *symname;
        if (!S_ISLNK(nd->dentry->d_inode->i_mode))
                return 0;
        if (nd->depth > MAX_SYMLINK_DEPTH)
                return -ELOOP;
        printd("Following symlink for dentry %p %s\n", nd->dentry,
               nd->dentry->d_name.name);
        nd->depth++;
        symname = nd->dentry->d_inode->i_op->readlink(nd->dentry);
        /* We need to pin in nd->dentry (the dentry of the symlink), since we need
         * it's symname's storage to stay in memory throughout the upcoming
         * link_path_walk().  The last_sym gets decreffed when we path_release() or
         * follow another symlink. */
        if (nd->last_sym)
                kref_put(&nd->last_sym->d_kref);
        kref_get(&nd->dentry->d_kref, 1);
        nd->last_sym = nd->dentry;
        /* If this an absolute path in the symlink, we need to free the old path and
         * start over, otherwise, we continue from the PARENT of nd (the symlink) */
        if (symname[0] == '/') {
                path_release(nd);
                if (!current)
                        nd->dentry = default_ns.root->mnt_root;
                else
                        nd->dentry = current->fs_env.root;      
                nd->mnt = nd->dentry->d_sb->s_mount;
                kref_get(&nd->mnt->mnt_kref, 1);
                kref_get(&nd->dentry->d_kref, 1);
        } else {
                climb_up(nd);
        }
        /* either way, keep on walking in the free world! */
        retval = link_path_walk(symname, nd);
        return (retval == 0 ? 1 : retval);
}

/* Little helper, to make it easier to break out of the nested loops.  Will also
 * '\0' out the first slash if it's slashes all the way down.  Or turtles. */
static bool packed_trailing_slashes(char *first_slash)
{
        for (char *i = first_slash; *i == '/'; i++) {
                if (*(i + 1) == '\0') {
                        *first_slash = '\0';
                        return TRUE;
                }
        }
        return FALSE;
}

/* Simple helper to set nd to track it's last name to be Name.  Also be careful
 * with the storage of name.  Don't use and nd's name past the lifetime of the
 * string used in the path_lookup()/link_path_walk/whatever.  Consider replacing
 * parts of this with a qstr builder.  Note this uses the dentry's d_op, which
 * might not be the dentry we care about. */
static void stash_nd_name(struct nameidata *nd, char *name)
{
        nd->last.name = name;
        nd->last.len = strlen(name);
        nd->last.hash = nd->dentry->d_op->d_hash(nd->dentry, &nd->last);
}

/* Resolves the links in a basic path walk.  0 for success, -EWHATEVER
 * otherwise.  The final lookup is returned via nd. */
static int link_path_walk(char *path, struct nameidata *nd)
{
        struct dentry *link_dentry;
        struct inode *link_inode, *nd_inode;
        char *next_slash;
        char *link = path;
        int error;

        /* Prevent crazy recursion */
        if (nd->depth > MAX_SYMLINK_DEPTH)
                return -ELOOP;
        /* skip all leading /'s */
        while (*link == '/')
                link++;
        /* if there's nothing left (null terminated), we're done.  This should only
         * happen for "/", which if we wanted a PARENT, should fail (there is no
         * parent). */
        if (*link == '\0') {
                if (nd->flags & LOOKUP_PARENT) {
                        set_errno(ENOENT);
                        return -1;
                }
                /* o/w, we're good */
                return 0;
        }
        /* iterate through each intermediate link of the path.  in general, nd
         * tracks where we are in the path, as far as dentries go.  once we have the
         * next dentry, we try to update nd based on that dentry.  link is the part
         * of the path string that we are looking up */
        while (1) {
                nd_inode = nd->dentry->d_inode;
                if ((error = check_perms(nd_inode, nd->intent)))
                        return error;
                /* find the next link, break out if it is the end */
                next_slash = strchr(link, '/');
                if (!next_slash) {
                        break;
                } else {
                        if (packed_trailing_slashes(next_slash)) {
                                nd->flags |= LOOKUP_DIRECTORY;
                                break;
                        }
                }
                /* skip over any interim ./ */
                if (!strncmp("./", link, 2))
                        goto next_loop;
                /* Check for "../", walk up */
                if (!strncmp("../", link, 3)) {
                        climb_up(nd);
                        goto next_loop;
                }
                *next_slash = '\0';
                link_dentry = do_lookup(nd->dentry, link);
                *next_slash = '/';
                if (!link_dentry)
                        return -ENOENT;
                /* make link_dentry the current step/answer */
                next_link(link_dentry, nd);
                kref_put(&link_dentry->d_kref); /* do_lookup gave us a refcnt dentry */
                /* we could be on a mountpoint or a symlink - need to follow them */
                follow_mount(nd);
                if ((error = follow_symlink(nd)) < 0)
                        return error;
                /* Turn off a possible DIRECTORY lookup, which could have been set
                 * during the follow_symlink (a symlink could have had a directory at
                 * the end), though it was in the middle of the real path. */
                nd->flags &= ~LOOKUP_DIRECTORY;
                if (!S_ISDIR(nd->dentry->d_inode->i_mode))
                        return -ENOTDIR;
next_loop:
                /* move through the path string to the next entry */
                link = next_slash + 1;
                /* advance past any other interim slashes.  we know we won't hit the end
                 * due to the for loop check above */
                while (*link == '/')
                        link++;
        }
        /* Now, we're on the last link of the path.  We need to deal with with . and
         * .. .  This might be weird with PARENT lookups - not sure what semantics
         * we want exactly.  This will give the parent of whatever the PATH was
         * supposed to look like.  Note that ND currently points to the parent of
         * the last item (link). */
        if (!strcmp(".", link)) {
                if (nd->flags & LOOKUP_PARENT) {
                        assert(nd->dentry->d_name.name);
                        stash_nd_name(nd, nd->dentry->d_name.name);
                        climb_up(nd);
                }
                return 0;
        }
        if (!strcmp("..", link)) {
                climb_up(nd);
                if (nd->flags & LOOKUP_PARENT) {
                        assert(nd->dentry->d_name.name);
                        stash_nd_name(nd, nd->dentry->d_name.name);
                        climb_up(nd);
                }
                return 0;
        }
        /* need to attempt to look it up, in case it's a symlink */
        link_dentry = do_lookup(nd->dentry, link);
        if (!link_dentry) {
                /* if there's no dentry, we are okay if we are looking for the parent */
                if (nd->flags & LOOKUP_PARENT) {
                        assert(strcmp(link, ""));
                        stash_nd_name(nd, link);
                        return 0;
                } else {
                        return -ENOENT;
                }
        }
        next_link(link_dentry, nd);
        kref_put(&link_dentry->d_kref); /* do_lookup gave us a refcnt'd dentry */
        /* at this point, nd is on the final link, but it might be a symlink */
        if (nd->flags & LOOKUP_FOLLOW) {
                error = follow_symlink(nd);
                if (error < 0)
                        return error;
                /* if we actually followed a symlink, then nd is set and we're done */
                if (error > 0)
                        return 0;
        }
        /* One way or another, nd is on the last element of the path, symlinks and
         * all.  Now we need to climb up to set nd back on the parent, if that's
         * what we wanted */
        if (nd->flags & LOOKUP_PARENT) {
                assert(nd->dentry->d_name.name);
                stash_nd_name(nd, link_dentry->d_name.name);
                climb_up(nd);
                return 0;
        }
        /* now, we have the dentry set, and don't want the parent, but might be on a
         * mountpoint still.  FYI: this hasn't been thought through completely. */
        follow_mount(nd);
        /* If we wanted a directory, but didn't get one, error out */
        if ((nd->flags & LOOKUP_DIRECTORY) && !S_ISDIR(nd->dentry->d_inode->i_mode))
                return -ENOTDIR;
        return 0;
}

/* Given path, return the inode for the final dentry.  The ND should be
 * initialized for the first call - specifically, we need the intent. 
 * LOOKUP_PARENT and friends go in the flags var, which is not the intent.
 *
 * If path_lookup wants a PARENT, but hits the top of the FS (root or
 * otherwise), we want it to error out.  It's still unclear how we want to
 * handle processes with roots that aren't root, but at the very least, we don't
 * want to think we have the parent of /, but have / itself.  Due to the way
 * link_path_walk works, if that happened, we probably don't have a
 * nd->last.name.  This needs more thought (TODO).
 *
 * Need to be careful too.  While the path has been copied-in to the kernel,
 * it's still user input.  */
int path_lookup(char *path, int flags, struct nameidata *nd)
{
        int retval;
        printd("Path lookup for %s\n", path);
        /* we allow absolute lookups with no process context */
        if (path[0] == '/') {                   /* absolute lookup */
                if (!current)
                        nd->dentry = default_ns.root->mnt_root;
                else
                        nd->dentry = current->fs_env.root;      
        } else {                                                /* relative lookup */
                assert(current);
                /* Don't need to lock on the fs_env since we're reading one item */
                nd->dentry = current->fs_env.pwd;       
        }
        nd->mnt = nd->dentry->d_sb->s_mount;
        /* Whenever references get put in the nd, incref them.  Whenever they are
         * removed, decref them. */
        kref_get(&nd->mnt->mnt_kref, 1);
        kref_get(&nd->dentry->d_kref, 1);
        nd->flags = flags;
        nd->depth = 0;                                  /* used in symlink following */
        retval =  link_path_walk(path, nd);     
        /* make sure our PARENT lookup worked */
        if (!retval && (flags & LOOKUP_PARENT))
                assert(nd->last.name);
        return retval;
}

/* Call this after any use of path_lookup when you are done with its results,
 * regardless of whether it succeeded or not.  It will free any references */
void path_release(struct nameidata *nd)
{
        kref_put(&nd->dentry->d_kref);
        kref_put(&nd->mnt->mnt_kref);
        /* Free the last symlink dentry used, if there was one */
        if (nd->last_sym) {
                kref_put(&nd->last_sym->d_kref);
                nd->last_sym = 0;                       /* catch reuse bugs */
        }
}

/* External version of mount, only call this after having a / mount */
int mount_fs(struct fs_type *fs, char *dev_name, char *path, int flags)
{
        struct nameidata nd_r = {0}, *nd = &nd_r;
        int retval = 0;
        retval = path_lookup(path, LOOKUP_DIRECTORY, nd);
        if (retval)
                goto out;
        /* taking the namespace of the vfsmount of path */ 
        if (!__mount_fs(fs, dev_name, nd->dentry, flags, nd->mnt->mnt_namespace))
                retval = -EINVAL;
out:
        path_release(nd);
        return retval;
}

/* Superblock functions */

/* Dentry "hash" function for the hash table to use.  Since we already have the
 * hash in the qstr, we don't need to rehash.  Also, note we'll be using the
 * dentry in question as both the key and the value. */
static size_t __dcache_hash(void *k)
{
        return (size_t)((struct dentry*)k)->d_name.hash;
}

/* Dentry cache hashtable equality function.  This means we need to pass in some
 * minimal dentry when doing a lookup. */
static ssize_t __dcache_eq(void *k1, void *k2)
{
        if (((struct dentry*)k1)->d_parent != ((struct dentry*)k2)->d_parent)
                return 0;
        /* TODO: use the FS-specific string comparison */
        return !strcmp(((struct dentry*)k1)->d_name.name,
                       ((struct dentry*)k2)->d_name.name);
}

/* Helper to alloc and initialize a generic superblock.  This handles all the
 * VFS related things, like lists.  Each FS will need to handle its own things
 * in it's *_get_sb(), usually involving reading off the disc. */
struct super_block *get_sb(void)
{
        struct super_block *sb = kmalloc(sizeof(struct super_block), 0);
        sb->s_dirty = FALSE;
        spinlock_init(&sb->s_lock);
        kref_init(&sb->s_kref, fake_release, 1); /* for the ref passed out */
        TAILQ_INIT(&sb->s_inodes);
        TAILQ_INIT(&sb->s_dirty_i);
        TAILQ_INIT(&sb->s_io_wb);
        TAILQ_INIT(&sb->s_lru_d);
        TAILQ_INIT(&sb->s_files);
        sb->s_dcache = create_hashtable(100, __dcache_hash, __dcache_eq);
        sb->s_icache = create_hashtable(100, __generic_hash, __generic_eq);
        spinlock_init(&sb->s_lru_lock);
        spinlock_init(&sb->s_dcache_lock);
        spinlock_init(&sb->s_icache_lock);
        sb->s_fs_info = 0; // can override somewhere else
        return sb;
}

/* Final stages of initializing a super block, including creating and linking
 * the root dentry, root inode, vmnt, and sb.  The d_op and root_ino are
 * FS-specific, but otherwise it's FS-independent, tricky, and not worth having
 * around multiple times.
 *
 * Not the world's best interface, so it's subject to change, esp since we're
 * passing (now 3) FS-specific things. */
void init_sb(struct super_block *sb, struct vfsmount *vmnt,
             struct dentry_operations *d_op, unsigned long root_ino,
             void *d_fs_info)
{
        /* Build and init the first dentry / inode.  The dentry ref is stored later
         * by vfsmount's mnt_root.  The parent is dealt with later. */
        struct dentry *d_root = get_dentry(sb, 0,  "/");        /* probably right */

        if (!d_root)
                panic("OOM!  init_sb() can't fail yet!");
        /* a lot of here on down is normally done in lookup() or create, since
         * get_dentry isn't a fully usable dentry.  The two FS-specific settings are
         * normally inherited from a parent within the same FS in get_dentry, but we
         * have none here. */
        d_root->d_op = d_op;
        d_root->d_fs_info = d_fs_info;
        struct inode *inode = get_inode(d_root);
        if (!inode)
                panic("This FS sucks!");
        inode->i_ino = root_ino;
        /* TODO: add the inode to the appropriate list (off i_list) */
        /* TODO: do we need to read in the inode?  can we do this on demand? */
        /* if this FS is already mounted, we'll need to do something different. */
        sb->s_op->read_inode(inode);
        icache_put(sb, inode);
        /* Link the dentry and SB to the VFS mount */
        vmnt->mnt_root = d_root;                                /* ref comes from get_dentry */
        vmnt->mnt_sb = sb;
        /* If there is no mount point, there is no parent.  This is true only for
         * the rootfs. */
        if (vmnt->mnt_mountpoint) {
                kref_get(&vmnt->mnt_mountpoint->d_kref, 1);     /* held by d_root */
                d_root->d_parent = vmnt->mnt_mountpoint;        /* dentry of the root */
        } else {
                d_root->d_parent = d_root;                      /* set root as its own parent */
        }
        /* insert the dentry into the dentry cache.  when's the earliest we can?
         * when's the earliest we should?  what about concurrent accesses to the
         * same dentry?  should be locking the dentry... */
        dcache_put(sb, d_root);
        kref_put(&inode->i_kref);               /* give up the ref from get_inode() */
}

/* Dentry Functions */

/* Helper to alloc and initialize a generic dentry.  The following needs to be
 * set still: d_op (if no parent), d_fs_info (opt), d_inode, connect the inode
 * to the dentry (and up the d_kref again), maybe dcache_put().  The inode
 * stitching is done in get_inode() or lookup (depending on the FS).
 * The setting of the d_op might be problematic when dealing with mounts.  Just
 * overwrite it.
 *
 * If the name is longer than the inline name, it will kmalloc a buffer, so
 * don't worry about the storage for *name after calling this. */
struct dentry *get_dentry(struct super_block *sb, struct dentry *parent,
                          char *name)
{
        assert(name);
        size_t name_len = strnlen(name, MAX_FILENAME_SZ);       /* not including \0! */
        struct dentry *dentry = kmem_cache_alloc(dentry_kcache, 0);
        char *l_name = 0;

        if (!dentry) {
                set_errno(ENOMEM);
                return 0;
        }
        //memset(dentry, 0, sizeof(struct dentry));
        kref_init(&dentry->d_kref, dentry_release, 1);  /* this ref is returned */
        spinlock_init(&dentry->d_lock);
        TAILQ_INIT(&dentry->d_subdirs);
        dentry->d_time = 0;
        kref_get(&sb->s_kref, 1);
        dentry->d_sb = sb;                                      /* storing a ref here... */
        dentry->d_mount_point = FALSE;
        dentry->d_mounted_fs = 0;
        if (parent)     {                                               /* no parent for rootfs mount */
                kref_get(&parent->d_kref, 1);
                dentry->d_op = parent->d_op;    /* d_op set in init_sb for parentless */
        }
        dentry->d_parent = parent;
        dentry->d_flags = DENTRY_USED;
        dentry->d_fs_info = 0;
        if (name_len < DNAME_INLINE_LEN) {
                strncpy(dentry->d_iname, name, name_len);
                dentry->d_iname[name_len] = '\0';
                qstr_builder(dentry, 0);
        } else {
                l_name = kmalloc(name_len + 1, 0);
                assert(l_name);
                strncpy(l_name, name, name_len);
                l_name[name_len] = '\0';
                qstr_builder(dentry, l_name);
        }
        /* Catch bugs by aggressively zeroing this (o/w we use old stuff) */
        dentry->d_inode = 0;
        return dentry;
}

/* Called when the dentry is unreferenced (after kref == 0).  This works closely
 * with the resurrection in dcache_get().
 *
 * The dentry is still in the dcache, but needs to be un-USED and added to the
 * LRU dentry list.  Even dentries that were used in a failed lookup need to be
 * cached - they ought to be the negative dentries.  Note that all dentries have
 * parents, even negative ones (it is needed to find it in the dcache). */
void dentry_release(struct kref *kref)
{
        struct dentry *dentry = container_of(kref, struct dentry, d_kref);

        printd("'Releasing' dentry %p: %s\n", dentry, dentry->d_name.name);
        /* DYING dentries (recently unlinked / rmdir'd) just get freed */
        if (dentry->d_flags & DENTRY_DYING) {
                __dentry_free(dentry);
                return;
        }
        /* This lock ensures the USED state and the TAILQ membership is in sync.
         * Also used to check the refcnt, though that might not be necessary. */
        spin_lock(&dentry->d_lock);
        /* While locked, we need to double check the kref, in case someone already
         * reup'd it.  Re-up? you're crazy!  Reee-up, you're outta yo mind! */
        if (!kref_refcnt(&dentry->d_kref)) {
                /* Note this is where negative dentries get set UNUSED */
                if (dentry->d_flags & DENTRY_USED) {
                        dentry->d_flags &= ~DENTRY_USED;
                        spin_lock(&dentry->d_sb->s_lru_lock);
                        TAILQ_INSERT_TAIL(&dentry->d_sb->s_lru_d, dentry, d_lru);
                        spin_unlock(&dentry->d_sb->s_lru_lock);
                } else {
                        /* and make sure it wasn't USED, then UNUSED again */
                        /* TODO: think about issues with this */
                        warn("This should be rare.  Tell brho this happened.");
                }
        }
        spin_unlock(&dentry->d_lock);
}

/* Called when we really dealloc and get rid of a dentry (like when it is
 * removed from the dcache, either for memory or correctness reasons)
 *
 * This has to handle two types of dentries: full ones (ones that had been used)
 * and ones that had been just for lookups - hence the check for d_inode.
 *
 * Note that dentries pin and kref their inodes.  When all the dentries are
 * gone, we want the inode to be released via kref.  The inode has internal /
 * weak references to the dentry, which are not refcounted. */
void __dentry_free(struct dentry *dentry)
{
        if (dentry->d_inode)
                printk("Freeing dentry %p: %s\n", dentry, dentry->d_name.name);
        assert(dentry->d_op);   /* catch bugs.  a while back, some lacked d_op */
        dentry->d_op->d_release(dentry);
        /* TODO: check/test the boundaries on this. */
        if (dentry->d_name.len > DNAME_INLINE_LEN)
                kfree((void*)dentry->d_name.name);
        kref_put(&dentry->d_sb->s_kref);
        if (dentry->d_parent)
                kref_put(&dentry->d_parent->d_kref);
        if (dentry->d_mounted_fs)
                kref_put(&dentry->d_mounted_fs->mnt_kref);
        if (dentry->d_inode) {
                TAILQ_REMOVE(&dentry->d_inode->i_dentry, dentry, d_alias);
                kref_put(&dentry->d_inode->i_kref);     /* dentries kref inodes */
        }
        kmem_cache_free(dentry_kcache, dentry);
}

/* Looks up the dentry for the given path, returning a refcnt'd dentry (or 0).
 * Permissions are applied for the current user, which is quite a broken system
 * at the moment.  Flags are lookup flags. */
struct dentry *lookup_dentry(char *path, int flags)
{
        struct dentry *dentry;
        struct nameidata nd_r = {0}, *nd = &nd_r;
        int error;

        error = path_lookup(path, flags, nd);
        if (error) {
                path_release(nd);
                set_errno(-error);
                return 0;
        }
        dentry = nd->dentry;
        kref_get(&dentry->d_kref, 1);
        path_release(nd);
        return dentry;
}

/* Get a dentry from the dcache.  At a minimum, we need the name hash and parent
 * in what_i_want, though most uses will probably be from a get_dentry() call.
 * We pass in the SB in the off chance that we don't want to use a get'd dentry.
 *
 * The unusual variable name (instead of just "key" or something) is named after
 * ex-SPC Castro's porn folder.  Caller deals with the memory for what_i_want.
 *
 * If the dentry is negative, we don't return the actual result - instead, we
 * set the negative flag in 'what i want'.  The reason is we don't want to
 * kref_get() and then immediately put (causing dentry_release()).  This also
 * means that dentry_release() should never get someone who wasn't USED (barring
 * the race, which it handles).  And we don't need to ever have a dentry set as
 * USED and NEGATIVE (which is always wrong, but would be needed for a cleaner
 * dentry_release()).
 *
 * This is where we do the "kref resurrection" - we are returning a kref'd
 * object, even if it wasn't kref'd before.  This means the dcache does NOT hold
 * krefs (it is a weak/internal ref), but it is a source of kref generation.  We
 * sync up with the possible freeing of the dentry by locking the table.  See
 * Doc/kref for more info. */
struct dentry *dcache_get(struct super_block *sb, struct dentry *what_i_want)
{
        struct dentry *found;
        /* This lock protects the hash, as well as ensures the returned object
         * doesn't get deleted/freed out from under us */
        spin_lock(&sb->s_dcache_lock);
        found = hashtable_search(sb->s_dcache, what_i_want);
        if (found) {
                if (found->d_flags & DENTRY_NEGATIVE) {
                        what_i_want->d_flags |= DENTRY_NEGATIVE;
                        spin_unlock(&sb->s_dcache_lock);
                        return 0;
                }
                spin_lock(&found->d_lock);
                __kref_get(&found->d_kref, 1);  /* prob could be done outside the lock*/
                /* If we're here (after kreffing) and it is not USED, we are the one who
                 * should resurrect */
                if (!(found->d_flags & DENTRY_USED)) {
                        found->d_flags |= DENTRY_USED;
                        spin_lock(&sb->s_lru_lock);
                        TAILQ_REMOVE(&sb->s_lru_d, found, d_lru);
                        spin_unlock(&sb->s_lru_lock);
                }
                spin_unlock(&found->d_lock);
        }
        spin_unlock(&sb->s_dcache_lock);
        return found;
}

/* Adds a dentry to the dcache.  Note the *dentry is both the key and the value.
 * If the value was already in there (which can happen iff it was negative), for
 * now we'll remove it and put the new one in there. */
void dcache_put(struct super_block *sb, struct dentry *key_val)
{
        struct dentry *old;
        int retval;
        spin_lock(&sb->s_dcache_lock);
        old = hashtable_remove(sb->s_dcache, key_val);
        if (old) {
                assert(old->d_flags & DENTRY_NEGATIVE);
                /* This is possible, but rare for now (about to be put on the LRU) */
                assert(!(old->d_flags & DENTRY_USED));
                assert(!kref_refcnt(&old->d_kref));
                spin_lock(&sb->s_lru_lock);
                TAILQ_REMOVE(&sb->s_lru_d, old, d_lru);
                spin_unlock(&sb->s_lru_lock);
                __dentry_free(old);
        }
        /* this returns 0 on failure (TODO: Fix this ghetto shit) */
        retval = hashtable_insert(sb->s_dcache, key_val, key_val);
        assert(retval);
        spin_unlock(&sb->s_dcache_lock);
}

/* Will remove and return the dentry.  Caller deallocs the key, but the retval
 * won't have a reference.  * Returns 0 if it wasn't found.  Callers can't
 * assume much - they should not use the reference they *get back*, (if they
 * already had one for key, they can use that).  There may be other users out
 * there. */
struct dentry *dcache_remove(struct super_block *sb, struct dentry *key)
{
        struct dentry *retval;
        spin_lock(&sb->s_dcache_lock);
        retval = hashtable_remove(sb->s_dcache, key);
        spin_unlock(&sb->s_dcache_lock);
        return retval;
}

/* This will clean out the LRU list, which are the unused dentries of the dentry
 * cache.  This will optionally only free the negative ones.  Note that we grab
 * the hash lock for the time we traverse the LRU list - this prevents someone
 * from getting a kref from the dcache, which could cause us trouble (we rip
 * someone off the list, who isn't unused, and they try to rip them off the
 * list). */
void dcache_prune(struct super_block *sb, bool negative_only)
{
        struct dentry *d_i, *temp;
        struct dentry_tailq victims = TAILQ_HEAD_INITIALIZER(victims);

        spin_lock(&sb->s_dcache_lock);
        spin_lock(&sb->s_lru_lock);
        TAILQ_FOREACH_SAFE(d_i, &sb->s_lru_d, d_lru, temp) {
                if (!(d_i->d_flags & DENTRY_USED)) {
                        if (negative_only && !(d_i->d_flags & DENTRY_NEGATIVE))
                                continue;
                        /* another place where we'd be better off with tools, not sol'ns */
                        hashtable_remove(sb->s_dcache, d_i);
                        TAILQ_REMOVE(&sb->s_lru_d, d_i, d_lru);
                        TAILQ_INSERT_HEAD(&victims, d_i, d_lru);
                }
        }
        spin_unlock(&sb->s_lru_lock);
        spin_unlock(&sb->s_dcache_lock);
        /* Now do the actual freeing, outside of the hash/LRU list locks.  This is
         * necessary since __dentry_free() will decref its parent, which may get
         * released and try to add itself to the LRU. */
        TAILQ_FOREACH_SAFE(d_i, &victims, d_lru, temp) {
                TAILQ_REMOVE(&victims, d_i, d_lru);
                assert(!kref_refcnt(&d_i->d_kref));
                __dentry_free(d_i);
        }
        /* It is possible at this point that there are new items on the LRU.  We
         * could loop back until that list is empty, if we care about this. */
}

/* Inode Functions */

/* Creates and initializes a new inode.  Generic fields are filled in.
 * FS-specific fields are filled in by the callout.  Specific fields are filled
 * in in read_inode() based on what's on the disk for a given i_no, or when the
 * inode is created (for new objects).
 *
 * i_no is set by the caller.  Note that this means this inode can be for an
 * inode that is already on disk, or it can be used when creating. */
struct inode *get_inode(struct dentry *dentry)
{
        struct super_block *sb = dentry->d_sb;
        /* FS allocs and sets the following: i_op, i_fop, i_pm.pm_op, and any FS
         * specific stuff. */
        struct inode *inode = sb->s_op->alloc_inode(sb);
        if (!inode) {
                set_errno(ENOMEM);
                return 0;
        }
        TAILQ_INSERT_HEAD(&sb->s_inodes, inode, i_sb_list);             /* weak inode ref */
        TAILQ_INIT(&inode->i_dentry);
        TAILQ_INSERT_TAIL(&inode->i_dentry, dentry, d_alias);   /* weak dentry ref*/
        /* one for the dentry->d_inode, one passed out */
        kref_init(&inode->i_kref, inode_release, 2);
        dentry->d_inode = inode;
        inode->i_ino = 0;                                       /* set by caller later */
        inode->i_blksize = sb->s_blocksize;
        spinlock_init(&inode->i_lock);
        kref_get(&sb->s_kref, 1);                       /* could allow the dentry to pin it */
        inode->i_sb = sb;
        inode->i_rdev = 0;                                      /* this has no real meaning yet */
        inode->i_bdev = sb->s_bdev;                     /* storing an uncounted ref */
        inode->i_state = 0;                                     /* need real states, like I_NEW */
        inode->dirtied_when = 0;
        inode->i_flags = 0;
        atomic_set(&inode->i_writecount, 0);
        /* Set up the page_map structures.  Default is to use the embedded one.
         * Might push some of this back into specific FSs.  For now, the FS tells us
         * what pm_op they want via i_pm.pm_op, which we set again in pm_init() */
        inode->i_mapping = &inode->i_pm;
        pm_init(inode->i_mapping, inode->i_pm.pm_op, inode);
        return inode;
}

/* Helper: loads/ reads in the inode numbered ino and attaches it to dentry */
void load_inode(struct dentry *dentry, unsigned long ino)
{
        struct inode *inode;

        /* look it up in the inode cache first */
        inode = icache_get(dentry->d_sb, ino);
        if (inode) {
                /* connect the dentry to its inode */
                TAILQ_INSERT_TAIL(&inode->i_dentry, dentry, d_alias);
                dentry->d_inode = inode;        /* storing the ref we got from icache_get */
                return;
        }
        /* otherwise, we need to do it manually */
        inode = get_inode(dentry);
        inode->i_ino = ino;
        dentry->d_sb->s_op->read_inode(inode);
        /* TODO: race here, two creators could miss in the cache, and then get here.
         * need a way to sync across a blocking call.  needs to be either at this
         * point in the code or per the ino (dentries could be different) */
        icache_put(dentry->d_sb, inode);
        kref_put(&inode->i_kref);
}

/* Helper op, used when creating regular files, directories, symlinks, etc.
 * Note we make a distinction between the mode and the file type (for now).
 * After calling this, call the FS specific version (create or mkdir), which
 * will set the i_ino, the filetype, and do any other FS-specific stuff.  Also
 * note that a lot of inode stuff was initialized in get_inode/alloc_inode.  The
 * stuff here is pertinent to the specific creator (user), mode, and time.  Also
 * note we don't pass this an nd, like Linux does... */
static struct inode *create_inode(struct dentry *dentry, int mode)
{
        /* note it is the i_ino that uniquely identifies a file in the specific
         * filesystem.  there's a diff between creating an inode (even for an in-use
         * ino) and then filling it in, and vs creating a brand new one.
         * get_inode() sets it to 0, and it should be filled in later in an
         * FS-specific manner. */
        struct inode *inode = get_inode(dentry);
        if (!inode)
                return 0;
        inode->i_mode = mode & S_PMASK; /* note that after this, we have no type */
        inode->i_nlink = 1;
        inode->i_size = 0;
        inode->i_blocks = 0;
        inode->i_atime.tv_sec = 0;              /* TODO: now! */
        inode->i_ctime.tv_sec = 0;
        inode->i_mtime.tv_sec = 0;
        inode->i_atime.tv_nsec = 0;             /* are these supposed to be the extra ns? */
        inode->i_ctime.tv_nsec = 0;
        inode->i_mtime.tv_nsec = 0;
        inode->i_bdev = inode->i_sb->s_bdev;
        /* when we have notions of users, do something here: */
        inode->i_uid = 0;
        inode->i_gid = 0;
        return inode;
}

/* Create a new disk inode in dir associated with dentry, with the given mode.
 * called when creating a regular file.  dir is the directory/parent.  dentry is
 * the dentry of the inode we are creating.  Note the lack of the nd... */
int create_file(struct inode *dir, struct dentry *dentry, int mode)
{
        struct inode *new_file = create_inode(dentry, mode);
        if (!new_file)
                return -1;
        dir->i_op->create(dir, dentry, mode, 0);
        icache_put(new_file->i_sb, new_file);
        kref_put(&new_file->i_kref);
        return 0;
}

/* Creates a new inode for a directory associated with dentry in dir with the
 * given mode. */
int create_dir(struct inode *dir, struct dentry *dentry, int mode)
{
        struct inode *new_dir = create_inode(dentry, mode);
        if (!new_dir)
                return -1;
        dir->i_op->mkdir(dir, dentry, mode);
        dir->i_nlink++;         /* Directories get a hardlink for every child dir */
        /* Make sure my parent tracks me.  This is okay, since no directory (dir)
         * can have more than one dentry */
        struct dentry *parent = TAILQ_FIRST(&dir->i_dentry);
        assert(parent && parent == TAILQ_LAST(&dir->i_dentry, dentry_tailq));
        /* parent dentry tracks dentry as a subdir, weak reference */
        TAILQ_INSERT_TAIL(&parent->d_subdirs, dentry, d_subdirs_link);
        icache_put(new_dir->i_sb, new_dir);
        kref_put(&new_dir->i_kref);
        return 0;
}

/* Creates a new inode for a symlink associated with dentry in dir, containing
 * the symlink symname */
int create_symlink(struct inode *dir, struct dentry *dentry,
                   const char *symname, int mode)
{
        struct inode *new_sym = create_inode(dentry, mode);
        if (!new_sym)
                return -1;
        dir->i_op->symlink(dir, dentry, symname);
        icache_put(new_sym->i_sb, new_sym);
        kref_put(&new_sym->i_kref);
        return 0;
}

/* Returns 0 if the given mode is acceptable for the inode, and an appropriate
 * error code if not.  Needs to be writen, based on some sensible rules, and
 * will also probably use 'current' */
int check_perms(struct inode *inode, int access_mode)
{
        return 0;       /* anything goes! */
}

/* Called after all external refs are gone to clean up the inode.  Once this is
 * called, all dentries pointing here are already done (one of them triggered
 * this via kref_put(). */
void inode_release(struct kref *kref)
{
        struct inode *inode = container_of(kref, struct inode, i_kref);
        TAILQ_REMOVE(&inode->i_sb->s_inodes, inode, i_sb_list);
        icache_remove(inode->i_sb, inode->i_ino);
        /* Might need to write back or delete the file/inode */
        if (inode->i_nlink) {
                if (inode->i_state & I_STATE_DIRTY)
                        inode->i_sb->s_op->write_inode(inode, TRUE);
        } else {
                inode->i_sb->s_op->delete_inode(inode);
        }
        if (S_ISFIFO(inode->i_mode)) {
                page_decref(kva2page(inode->i_pipe->p_buf));
                kfree(inode->i_pipe);
        }
        /* TODO: (BDEV) */
        // kref_put(inode->i_bdev->kref); /* assuming it's a bdev, could be a pipe*/
        /* Either way, we dealloc the in-memory version */
        inode->i_sb->s_op->dealloc_inode(inode);        /* FS-specific clean-up */
        kref_put(&inode->i_sb->s_kref);
        /* TODO: clean this up */
        assert(inode->i_mapping == &inode->i_pm);
        kmem_cache_free(inode_kcache, inode);
}

/* Fills in kstat with the stat information for the inode */
void stat_inode(struct inode *inode, struct kstat *kstat)
{
        kstat->st_dev = inode->i_sb->s_dev;
        kstat->st_ino = inode->i_ino;
        kstat->st_mode = inode->i_mode;
        kstat->st_nlink = inode->i_nlink;
        kstat->st_uid = inode->i_uid;
        kstat->st_gid = inode->i_gid;
        kstat->st_rdev = inode->i_rdev;
        kstat->st_size = inode->i_size;
        kstat->st_blksize = inode->i_blksize;
        kstat->st_blocks = inode->i_blocks;
        kstat->st_atime = inode->i_atime;
        kstat->st_mtime = inode->i_mtime;
        kstat->st_ctime = inode->i_ctime;
}

/* Inode Cache management.  In general, search on the ino, get a refcnt'd value
 * back.  Remove does not give you a reference back - it should only be called
 * in inode_release(). */
struct inode *icache_get(struct super_block *sb, unsigned long ino)
{
        /* This is the same style as in pid2proc, it's the "safely create a strong
         * reference from a weak one, so long as other strong ones exist" pattern */
        spin_lock(&sb->s_icache_lock);
        struct inode *inode = hashtable_search(sb->s_icache, (void*)ino);
        if (inode)
                if (!kref_get_not_zero(&inode->i_kref, 1))
                        inode = 0;
        spin_unlock(&sb->s_icache_lock);
        return inode;
}

void icache_put(struct super_block *sb, struct inode *inode)
{
        spin_lock(&sb->s_icache_lock);
        /* there's a race in load_ino() that could trigger this */
        assert(!hashtable_search(sb->s_icache, (void*)inode->i_ino));
        hashtable_insert(sb->s_icache, (void*)inode->i_ino, inode);
        spin_unlock(&sb->s_icache_lock);
}

struct inode *icache_remove(struct super_block *sb, unsigned long ino)
{
        struct inode *inode;
        /* Presumably these hashtable removals could be easier since callers
         * actually know who they are (same with the pid2proc hash) */
        spin_lock(&sb->s_icache_lock);
        inode = hashtable_remove(sb->s_icache, (void*)ino);
        spin_unlock(&sb->s_icache_lock);
        assert(inode && !kref_refcnt(&inode->i_kref));
        return inode;
}

/* File functions */

/* Read count bytes from the file into buf, starting at *offset, which is
 * increased accordingly, returning the number of bytes transfered.  Most
 * filesystems will use this function for their f_op->read.
 * Note, this uses the page cache. */
ssize_t generic_file_read(struct file *file, char *buf, size_t count,
                          off64_t *offset)
{
        struct page *page;
        int error;
        off64_t page_off;
        unsigned long first_idx, last_idx;
        size_t copy_amt;
        char *buf_end;

        /* Consider pushing some error checking higher in the VFS */
        if (!count)
                return 0;
        if (*offset == file->f_dentry->d_inode->i_size)
                return 0; /* EOF */
        /* Make sure we don't go past the end of the file */
        if (*offset + count > file->f_dentry->d_inode->i_size) {
                count = file->f_dentry->d_inode->i_size - *offset;
        }
        page_off = *offset & (PGSIZE - 1);
        first_idx = *offset >> PGSHIFT;
        last_idx = (*offset + count) >> PGSHIFT;
        buf_end = buf + count;
        /* For each file page, make sure it's in the page cache, then copy it out.
         * TODO: will probably need to consider concurrently truncated files here.*/
        for (int i = first_idx; i <= last_idx; i++) {
                error = pm_load_page(file->f_mapping, i, &page);
                assert(!error); /* TODO: handle ENOMEM and friends */
                copy_amt = MIN(PGSIZE - page_off, buf_end - buf);
                /* TODO: (UMEM) think about this.  if it's a user buffer, we're relying
                 * on current to detect whose it is (which should work for async calls).
                 * Also, need to propagate errors properly...  Probably should do a
                 * user_mem_check, then free, and also to make a distinction between
                 * when the kernel wants a read/write (TODO: KFOP) */
                if (current) {
                        memcpy_to_user(current, buf, page2kva(page) + page_off, copy_amt);
                } else {
                        memcpy(buf, page2kva(page) + page_off, copy_amt);
                }
                buf += copy_amt;
                page_off = 0;
                page_decref(page);      /* it's still in the cache, we just don't need it */
        }
        assert(buf == buf_end);
        *offset += count;
        return count;
}

/* Write count bytes from buf to the file, starting at *offset, which is
 * increased accordingly, returning the number of bytes transfered.  Most
 * filesystems will use this function for their f_op->write.  Note, this uses
 * the page cache.
 *
 * Changes don't get flushed to disc til there is an fsync, page cache eviction,
 * or other means of trying to writeback the pages. */
ssize_t generic_file_write(struct file *file, const char *buf, size_t count,
                           off64_t *offset)
{
        struct page *page;
        int error;
        off64_t page_off;
        unsigned long first_idx, last_idx;
        size_t copy_amt;
        const char *buf_end;

        /* Consider pushing some error checking higher in the VFS */
        if (!count)
                return 0;
        /* Extend the file.  Should put more checks in here, and maybe do this per
         * page in the for loop below. */
        if (*offset + count > file->f_dentry->d_inode->i_size)
                file->f_dentry->d_inode->i_size = *offset + count;
        page_off = *offset & (PGSIZE - 1);
        first_idx = *offset >> PGSHIFT;
        last_idx = (*offset + count) >> PGSHIFT;
        buf_end = buf + count;
        /* For each file page, make sure it's in the page cache, then write it.*/
        for (int i = first_idx; i <= last_idx; i++) {
                error = pm_load_page(file->f_mapping, i, &page);
                assert(!error); /* TODO: handle ENOMEM and friends */
                copy_amt = MIN(PGSIZE - page_off, buf_end - buf);
                /* TODO: (UMEM) (KFOP) think about this.  if it's a user buffer, we're
                 * relying on current to detect whose it is (which should work for async
                 * calls). */
                if (current) {
                        memcpy_from_user(current, page2kva(page) + page_off, buf, copy_amt);
                } else {
                        memcpy(page2kva(page) + page_off, buf, copy_amt);
                }
                buf += copy_amt;
                page_off = 0;
                page_decref(page);      /* it's still in the cache, we just don't need it */
        }
        assert(buf == buf_end);
        *offset += count;
        return count;
}

/* Directories usually use this for their read method, which is the way glibc
 * currently expects us to do a readdir (short of doing linux's getdents).  Will
 * probably need work, based on whatever real programs want. */
ssize_t generic_dir_read(struct file *file, char *u_buf, size_t count,
                         off64_t *offset)
{
        struct kdirent dir_r = {0}, *dirent = &dir_r;
        int retval = 1;
        size_t amt_copied = 0;
        char *buf_end = u_buf + count;

        if (!S_ISDIR(file->f_dentry->d_inode->i_mode)) {
                set_errno(ENOTDIR);
                return -1;
        }
        if (!count)
                return 0;
        /* start readdir from where it left off: */
        dirent->d_off = *offset;
        for (   ;
                u_buf + sizeof(struct kdirent) <= buf_end;
                u_buf += sizeof(struct kdirent)) {
                /* TODO: UMEM/KFOP (pin the u_buf in the syscall, ditch the local copy,
                 * get rid of this memcpy and reliance on current, etc).  Might be
                 * tricky with the dirent->d_off and trust issues */
                retval = file->f_op->readdir(file, dirent);
                if (retval < 0) {
                        set_errno(-retval);
                        break;
                }
                /* Slight info exposure: could be extra crap after the name in the
                 * dirent (like the name of a deleted file) */
                if (current) {
                        memcpy_to_user(current, u_buf, dirent, sizeof(struct dirent));
                } else {
                        memcpy(u_buf, dirent, sizeof(struct dirent));
                }
                amt_copied += sizeof(struct dirent);
                /* 0 signals end of directory */
                if (retval == 0)
                        break;
        }
        /* Next time read is called, we pick up where we left off */
        *offset = dirent->d_off;        /* UMEM */
        /* important to tell them how much they got.  they often keep going til they
         * get 0 back (in the case of ls).  it's also how much has been read, but it
         * isn't how much the f_pos has moved (which is opaque to the VFS). */
        return amt_copied;
}

/* Opens the file, using permissions from current for lack of a better option.
 * It will attempt to create the file if it does not exist and O_CREAT is
 * specified.  This will return 0 on failure, and set errno.  TODO: There's some
 * stuff that we don't do, esp related file truncating/creation.  flags are for
 * opening, the mode is for creating.  The flags related to how to create
 * (O_CREAT_FLAGS) are handled in this function, not in create_file().
 *
 * It's tempting to split this into a do_file_create and a do_file_open, based
 * on the O_CREAT flag, but the O_CREAT flag can be ignored if the file exists
 * already and O_EXCL isn't specified.  We could have open call create if it
 * fails, but for now we'll keep it as is. */
struct file *do_file_open(char *path, int flags, int mode)
{
        struct file *file = 0;
        struct dentry *file_d;
        struct inode *parent_i;
        struct nameidata nd_r = {0}, *nd = &nd_r;
        int error;

        /* The file might exist, lets try to just open it right away */
        nd->intent = LOOKUP_OPEN;
        error = path_lookup(path, LOOKUP_FOLLOW, nd);
        if (!error) {
                /* Still need to make sure we didn't want to O_EXCL create */
                if ((flags & O_CREAT) && (flags & O_EXCL)) {
                        set_errno(EEXIST);
                        goto out_path_only;
                }
                file_d = nd->dentry;
                kref_get(&file_d->d_kref, 1);
                goto open_the_file;
        }
        /* So it didn't already exist, release the path from the previous lookup,
         * and then we try to create it. */
        path_release(nd);       
        /* get the parent, following links.  this means you get the parent of the
         * final link (which may not be in 'path' in the first place. */
        nd->intent = LOOKUP_CREATE;
        error = path_lookup(path, LOOKUP_PARENT | LOOKUP_FOLLOW, nd);
        if (error) {
                set_errno(-error);
                goto out_path_only;
        }
        /* see if the target is there (shouldn't be), and handle accordingly */
        file_d = do_lookup(nd->dentry, nd->last.name); 
        if (!file_d) {
                if (!(flags & O_CREAT)) {
                        set_errno(ENOENT);
                        goto out_path_only;
                }
                /* Create the inode/file.  get a fresh dentry too: */
                file_d = get_dentry(nd->dentry->d_sb, nd->dentry, nd->last.name);
                if (!file_d)
                        goto out_path_only;
                parent_i = nd->dentry->d_inode;
                /* Note that the mode technically should only apply to future opens,
                 * but we apply it immediately. */
                if (create_file(parent_i, file_d, mode))        /* sets errno */
                        goto out_file_d;
                dcache_put(file_d->d_sb, file_d);
        } else {        /* something already exists */
                /* this can happen due to concurrent access, but needs to be thought
                 * through */
                panic("File shouldn't be here!");
                if ((flags & O_CREAT) && (flags & O_EXCL)) {
                        /* wanted to create, not open, bail out */
                        set_errno(EEXIST);
                        goto out_file_d;
                }
        }
open_the_file:
        /* now open the file (freshly created or if it already existed).  At this
         * point, file_d is a refcnt'd dentry, regardless of which branch we took.*/
        if (flags & O_TRUNC) {
                file_d->d_inode->i_size = 0;
                /* TODO: probably should remove the garbage pages from the page map */
        }
        file = dentry_open(file_d, flags);                              /* sets errno */
        /* Note the fall through to the exit paths.  File is 0 by default and if
         * dentry_open fails. */
out_file_d:
        kref_put(&file_d->d_kref);
out_path_only:
        path_release(nd);
        return file;
}

/* Path is the location of the symlink, sometimes called the "new path", and
 * symname is who we link to, sometimes called the "old path". */
int do_symlink(char *path, const char *symname, int mode)
{
        struct dentry *sym_d;
        struct inode *parent_i;
        struct nameidata nd_r = {0}, *nd = &nd_r;
        int error;
        int retval = -1;

        nd->intent = LOOKUP_CREATE;
        /* get the parent, but don't follow links */
        error = path_lookup(path, LOOKUP_PARENT, nd);
        if (error) {
                set_errno(-error);
                goto out_path_only;
        }
        /* see if the target is already there, handle accordingly */
        sym_d = do_lookup(nd->dentry, nd->last.name); 
        if (sym_d) {
                set_errno(EEXIST);
                goto out_sym_d;
        }
        /* Doesn't already exist, let's try to make it: */
        sym_d = get_dentry(nd->dentry->d_sb, nd->dentry, nd->last.name);
        if (!sym_d)
                goto out_path_only;
        parent_i = nd->dentry->d_inode;
        if (create_symlink(parent_i, sym_d, symname, mode))
                goto out_sym_d;
        dcache_put(sym_d->d_sb, sym_d);
        retval = 0;                             /* Note the fall through to the exit paths */
out_sym_d:
        kref_put(&sym_d->d_kref);
out_path_only:
        path_release(nd);
        return retval;
}

/* Makes a hard link for the file behind old_path to new_path */
int do_link(char *old_path, char *new_path)
{
        struct dentry *link_d, *old_d;
        struct inode *inode, *parent_dir;
        struct nameidata nd_r = {0}, *nd = &nd_r;
        int error;
        int retval = -1;

        nd->intent = LOOKUP_CREATE;
        /* get the absolute parent of the new_path */
        error = path_lookup(new_path, LOOKUP_PARENT | LOOKUP_FOLLOW, nd);
        if (error) {
                set_errno(-error);
                goto out_path_only;
        }
        parent_dir = nd->dentry->d_inode;
        /* see if the new target is already there, handle accordingly */
        link_d = do_lookup(nd->dentry, nd->last.name); 
        if (link_d) {
                set_errno(EEXIST);
                goto out_link_d;
        }
        /* Doesn't already exist, let's try to make it.  Still need to stitch it to
         * an inode and set its FS-specific stuff after this.*/
        link_d = get_dentry(nd->dentry->d_sb, nd->dentry, nd->last.name);
        if (!link_d)
                goto out_path_only;
        /* Now let's get the old_path target */
        old_d = lookup_dentry(old_path, LOOKUP_FOLLOW);
        if (!old_d)                                     /* errno set by lookup_dentry */
                goto out_link_d;
        /* For now, can only link to files */
        if (!S_ISREG(old_d->d_inode->i_mode)) {
                set_errno(EPERM);
                goto out_both_ds;
        }
        /* Must be on the same FS */
        if (old_d->d_sb != link_d->d_sb) {
                set_errno(EXDEV);
                goto out_both_ds;
        }
        /* Do whatever FS specific stuff there is first (which is also a chance to
         * bail out). */
        error = parent_dir->i_op->link(old_d, parent_dir, link_d);
        if (error) {
                set_errno(-error);
                goto out_both_ds;
        }
        /* Finally stitch it up */
        inode = old_d->d_inode;
        kref_get(&inode->i_kref, 1);
        link_d->d_inode = inode;
        inode->i_nlink++;
        TAILQ_INSERT_TAIL(&inode->i_dentry, link_d, d_alias);   /* weak ref */
        dcache_put(link_d->d_sb, link_d);
        retval = 0;                             /* Note the fall through to the exit paths */
out_both_ds:
        kref_put(&old_d->d_kref);
out_link_d:
        kref_put(&link_d->d_kref);
out_path_only:
        path_release(nd);
        return retval;
}

/* Unlinks path from the directory tree.  Read the Documentation for more info.
 */
int do_unlink(char *path)
{
        struct dentry *dentry;
        struct inode *parent_dir;
        struct nameidata nd_r = {0}, *nd = &nd_r;
        int error;
        int retval = -1;

        /* get the parent of the target, and don't follow a final link */
        error = path_lookup(path, LOOKUP_PARENT, nd);
        if (error) {
                set_errno(-error);
                goto out_path_only;
        }
        parent_dir = nd->dentry->d_inode;
        /* make sure the target is there */
        dentry = do_lookup(nd->dentry, nd->last.name); 
        if (!dentry) {
                set_errno(ENOENT);
                goto out_path_only;
        }
        /* Make sure the target is not a directory */
        if (S_ISDIR(dentry->d_inode->i_mode)) {
                set_errno(EISDIR);
                goto out_dentry;
        }
        /* Remove the dentry from its parent */
        error = parent_dir->i_op->unlink(parent_dir, dentry);
        if (error) {
                set_errno(-error);
                goto out_dentry;
        }
        /* Now that our parent doesn't track us, we need to make sure we aren't
         * findable via the dentry cache.  DYING, so we will be freed in
         * dentry_release() */
        dentry->d_flags |= DENTRY_DYING;
        dcache_remove(dentry->d_sb, dentry);
        dentry->d_inode->i_nlink--;     /* TODO: race here, esp with a decref */
        /* At this point, the dentry is unlinked from the FS, and the inode has one
         * less link.  When the in-memory objects (dentry, inode) are going to be
         * released (after all open files are closed, and maybe after entries are
         * evicted from the cache), then nlinks will get checked and the FS-file
         * will get removed from the disk */
        retval = 0;                             /* Note the fall through to the exit paths */
out_dentry:
        kref_put(&dentry->d_kref);
out_path_only:
        path_release(nd);
        return retval;
}

/* Checks to see if path can be accessed via mode.  Need to actually send the
 * mode along somehow, so this doesn't do much now.  This is an example of
 * decent error propagation from the lower levels via int retvals. */
int do_access(char *path, int mode)
{
        struct nameidata nd_r = {0}, *nd = &nd_r;
        int retval = 0;
        nd->intent = LOOKUP_ACCESS;
        retval = path_lookup(path, 0, nd);
        path_release(nd);       
        return retval;
}

int do_chmod(char *path, int mode)
{
        struct nameidata nd_r = {0}, *nd = &nd_r;
        int retval = 0;
        retval = path_lookup(path, 0, nd);
        if (!retval) {
                #if 0
                /* TODO: when we have notions of uid, check for the proc's uid */
                if (nd->dentry->d_inode->i_uid != UID_OF_ME)
                        retval = -EPERM;
                else
                #endif
                        nd->dentry->d_inode->i_mode |= mode & S_PMASK;
        }
        path_release(nd);       
        return retval;
}

/* Make a directory at path with mode.  Returns -1 and sets errno on errors */
int do_mkdir(char *path, int mode)
{
        struct dentry *dentry;
        struct inode *parent_i;
        struct nameidata nd_r = {0}, *nd = &nd_r;
        int error;
        int retval = -1;

        nd->intent = LOOKUP_CREATE;
        /* get the parent, but don't follow links */
        error = path_lookup(path, LOOKUP_PARENT, nd);
        if (error) {
                set_errno(-error);
                goto out_path_only;
        }
        /* see if the target is already there, handle accordingly */
        dentry = do_lookup(nd->dentry, nd->last.name); 
        if (dentry) {
                set_errno(EEXIST);
                goto out_dentry;
        }
        /* Doesn't already exist, let's try to make it: */
        dentry = get_dentry(nd->dentry->d_sb, nd->dentry, nd->last.name);
        if (!dentry)
                goto out_path_only;
        parent_i = nd->dentry->d_inode;
        if (create_dir(parent_i, dentry, mode))
                goto out_dentry;
        dcache_put(dentry->d_sb, dentry);
        retval = 0;                             /* Note the fall through to the exit paths */
out_dentry:
        kref_put(&dentry->d_kref);
out_path_only:
        path_release(nd);
        return retval;
}

int do_rmdir(char *path)
{
        struct dentry *dentry;
        struct inode *parent_i;
        struct nameidata nd_r = {0}, *nd = &nd_r;
        int error;
        int retval = -1;

        /* get the parent, following links (probably want this), and we must get a
         * directory.  Note, current versions of path_lookup can't handle both
         * PARENT and DIRECTORY, at least, it doesn't check that *path is a
         * directory. */
        error = path_lookup(path, LOOKUP_PARENT | LOOKUP_FOLLOW | LOOKUP_DIRECTORY,
                            nd);
        if (error) {
                set_errno(-error);
                goto out_path_only;
        }
        /* make sure the target is already there, handle accordingly */
        dentry = do_lookup(nd->dentry, nd->last.name); 
        if (!dentry) {
                set_errno(ENOENT);
                goto out_path_only;
        }
        if (!S_ISDIR(dentry->d_inode->i_mode)) {
                set_errno(ENOTDIR);
                goto out_dentry;
        }
        if (dentry->d_mount_point) {
                set_errno(EBUSY);
                goto out_dentry;
        }
        /* TODO: make sure we aren't a mount or processes root (EBUSY) */
        /* Now for the removal.  the FSs will check if they are empty */
        parent_i = nd->dentry->d_inode;
        error = parent_i->i_op->rmdir(parent_i, dentry);
        if (error < 0) {
                set_errno(-error);
                goto out_dentry;
        }
        /* Now that our parent doesn't track us, we need to make sure we aren't
         * findable via the dentry cache.  DYING, so we will be freed in
         * dentry_release() */
        dentry->d_flags |= DENTRY_DYING;
        dcache_remove(dentry->d_sb, dentry);
        /* Decref ourselves, so inode_release() knows we are done */
        dentry->d_inode->i_nlink--;
        TAILQ_REMOVE(&nd->dentry->d_subdirs, dentry, d_subdirs_link);
        parent_i->i_nlink--;            /* TODO: race on this, esp since its a decref */
        /* we still have d_parent and a kref on our parent, which will go away when
         * the in-memory dentry object goes away. */
        retval = 0;                             /* Note the fall through to the exit paths */
out_dentry:
        kref_put(&dentry->d_kref);
out_path_only:
        path_release(nd);
        return retval;
}

/* Pipes: Doing a simple buffer with reader and writer offsets.  Size is power
 * of two, so we can easily compute its status and whatnot. */

#define PIPE_SZ                                 (1 << PGSHIFT)

static size_t pipe_get_rd_idx(struct pipe_inode_info *pii)
{
        return pii->p_rd_off & (PIPE_SZ - 1);
}

static size_t pipe_get_wr_idx(struct pipe_inode_info *pii)
{

        return pii->p_wr_off & (PIPE_SZ - 1);
}

static bool pipe_is_empty(struct pipe_inode_info *pii)
{
        return __ring_empty(pii->p_wr_off, pii->p_rd_off);
}

static bool pipe_is_full(struct pipe_inode_info *pii)
{
        return __ring_full(PIPE_SZ, pii->p_wr_off, pii->p_rd_off);
}

static size_t pipe_nr_full(struct pipe_inode_info *pii)
{
        return __ring_nr_full(pii->p_wr_off, pii->p_rd_off);
}

static size_t pipe_nr_empty(struct pipe_inode_info *pii)
{
        return __ring_nr_empty(PIPE_SZ, pii->p_wr_off, pii->p_rd_off);
}

ssize_t pipe_file_read(struct file *file, char *buf, size_t count,
                       off64_t *offset)
{
        struct pipe_inode_info *pii = file->f_dentry->d_inode->i_pipe;
        size_t copy_amt, amt_copied = 0;

        cv_lock(&pii->p_cv);
        while (pipe_is_empty(pii)) {
                /* We wait til the pipe is drained before sending EOF if there are no
                 * writers (instead of aborting immediately) */
                if (!pii->p_nr_writers) {
                        cv_unlock(&pii->p_cv);
                        return 0;
                }
                if (file->f_flags & O_NONBLOCK) {
                        cv_unlock(&pii->p_cv);
                        set_errno(EAGAIN);
                        return -1;
                }
                cv_wait(&pii->p_cv);
                cpu_relax();
        }
        /* We might need to wrap-around with our copy, so we'll do the copy in two
         * passes.  This will copy up to the end of the buffer, then on the next
         * pass will copy the rest to the beginning of the buffer (if necessary) */
        for (int i = 0; i < 2; i++) {
                copy_amt = MIN(PIPE_SZ - pipe_get_rd_idx(pii),
                               MIN(pipe_nr_full(pii), count));
                assert(current);        /* shouldn't pipe from the kernel */
                memcpy_to_user(current, buf, pii->p_buf + pipe_get_rd_idx(pii),
                               copy_amt);
                buf += copy_amt;
                count -= copy_amt;
                pii->p_rd_off += copy_amt;
                amt_copied += copy_amt;
        }
        /* Just using one CV for both readers and writers.  We should rarely have
         * multiple readers or writers. */
        if (amt_copied)
                __cv_broadcast(&pii->p_cv);
        cv_unlock(&pii->p_cv);
        return amt_copied;
}

/* Note: we're not dealing with PIPE_BUF and minimum atomic chunks, unless I
 * have to later. */
ssize_t pipe_file_write(struct file *file, const char *buf, size_t count,
                        off64_t *offset)
{
        struct pipe_inode_info *pii = file->f_dentry->d_inode->i_pipe;
        size_t copy_amt, amt_copied = 0;

        cv_lock(&pii->p_cv);
        /* Write aborts right away if there are no readers, regardless of pipe
         * status. */
        if (!pii->p_nr_readers) {
                cv_unlock(&pii->p_cv);
                set_errno(EPIPE);
                return -1;
        }
        while (pipe_is_full(pii)) {
                if (file->f_flags & O_NONBLOCK) {
                        cv_unlock(&pii->p_cv);
                        set_errno(EAGAIN);
                        return -1;
                }
                cv_wait(&pii->p_cv);
                cpu_relax();
                /* Still need to check in the loop, in case the last reader left while
                 * we slept. */
                if (!pii->p_nr_readers) {
                        cv_unlock(&pii->p_cv);
                        set_errno(EPIPE);
                        return -1;
                }
        }
        /* We might need to wrap-around with our copy, so we'll do the copy in two
         * passes.  This will copy up to the end of the buffer, then on the next
         * pass will copy the rest to the beginning of the buffer (if necessary) */
        for (int i = 0; i < 2; i++) {
                copy_amt = MIN(PIPE_SZ - pipe_get_wr_idx(pii),
                               MIN(pipe_nr_empty(pii), count));
                assert(current);        /* shouldn't pipe from the kernel */
                memcpy_from_user(current, pii->p_buf + pipe_get_wr_idx(pii), buf,
                                 copy_amt);
                buf += copy_amt;
                count -= copy_amt;
                pii->p_wr_off += copy_amt;
                amt_copied += copy_amt;
        }
        /* Just using one CV for both readers and writers.  We should rarely have
         * multiple readers or writers. */
        if (amt_copied)
                __cv_broadcast(&pii->p_cv);
        cv_unlock(&pii->p_cv);
        return amt_copied;
}

/* In open and release, we need to track the number of readers and writers,
 * which we can differentiate by the file flags. */
int pipe_open(struct inode *inode, struct file *file)
{
        struct pipe_inode_info *pii = inode->i_pipe;
        cv_lock(&pii->p_cv);
        /* Ugliness due to not using flags for O_RDONLY and friends... */
        if (file->f_mode == S_IRUSR) {
                pii->p_nr_readers++;
        } else if (file->f_mode == S_IWUSR) {
                pii->p_nr_writers++;
        } else {
                warn("Bad pipe file flags 0x%x\n", file->f_flags);
        }
        cv_unlock(&pii->p_cv);
        return 0;
}

int pipe_release(struct inode *inode, struct file *file)
{
        struct pipe_inode_info *pii = inode->i_pipe;
        cv_lock(&pii->p_cv);
        /* Ugliness due to not using flags for O_RDONLY and friends... */
        if (file->f_mode == S_IRUSR) {
                pii->p_nr_readers--;
        } else if (file->f_mode == S_IWUSR) {
                pii->p_nr_writers--;
        } else {
                warn("Bad pipe file flags 0x%x\n", file->f_flags);
        }
        /* need to wake up any sleeping readers/writers, since we might be done */
        __cv_broadcast(&pii->p_cv);
        cv_unlock(&pii->p_cv);
        return 0;
}

struct file_operations pipe_f_op = {
        .read = pipe_file_read,
        .write = pipe_file_write,
        .open = pipe_open,
        .release = pipe_release,
        0
};

void pipe_debug(struct file *f)
{
        struct pipe_inode_info *pii = f->f_dentry->d_inode->i_pipe;
        assert(pii);
        printk("PIPE %p\n", pii);
        printk("\trdoff %p\n", pii->p_rd_off);
        printk("\twroff %p\n", pii->p_wr_off);
        printk("\tnr_rds %d\n", pii->p_nr_readers);
        printk("\tnr_wrs %d\n", pii->p_nr_writers);
        printk("\tcv waiters %d\n", pii->p_cv.nr_waiters);

}

/* General plan: get a dentry/inode to represent the pipe.  We'll alloc it from
 * the default_ns SB, but won't actually link it anywhere.  It'll only be held
 * alive by the krefs, til all the FDs are closed. */
int do_pipe(struct file **pipe_files, int flags)
{
        struct dentry *pipe_d;
        struct inode *pipe_i;
        struct file *pipe_f_read, *pipe_f_write;
        struct super_block *def_sb = default_ns.root->mnt_sb;
        struct pipe_inode_info *pii;

        pipe_d = get_dentry(def_sb, 0, "pipe");
        if (!pipe_d)
                return -1;
        pipe_d->d_op = &dummy_d_op;
        pipe_i = get_inode(pipe_d);
        if (!pipe_i)
                goto error_post_dentry;
        /* preemptively mark the dentry for deletion.  we have an unlinked dentry
         * right off the bat, held in only by the kref chain (pipe_d is the ref). */
        pipe_d->d_flags |= DENTRY_DYING;
        /* pipe_d->d_inode still has one ref to pipe_i, keeping the inode alive */
        kref_put(&pipe_i->i_kref);
        /* init inode fields.  note we're using the dummy ops for i_op and d_op */
        pipe_i->i_mode = S_IRWXU | S_IRWXG | S_IRWXO;
        SET_FTYPE(pipe_i->i_mode, __S_IFIFO);   /* using type == FIFO */
        pipe_i->i_nlink = 1;                    /* one for the dentry */
        pipe_i->i_uid = 0;
        pipe_i->i_gid = 0;
        pipe_i->i_size = PGSIZE;
        pipe_i->i_blocks = 0;
        pipe_i->i_atime.tv_sec = 0;
        pipe_i->i_atime.tv_nsec = 0;
        pipe_i->i_mtime.tv_sec = 0;
        pipe_i->i_mtime.tv_nsec = 0;
        pipe_i->i_ctime.tv_sec = 0;
        pipe_i->i_ctime.tv_nsec = 0;
        pipe_i->i_fs_info = 0;
        pipe_i->i_op = &dummy_i_op;
        pipe_i->i_fop = &pipe_f_op;
        pipe_i->i_socket = FALSE;
        /* Actually build the pipe.  We're using one page, hanging off the
         * pipe_inode_info struct.  When we release the inode, we free the pipe
         * memory too */
        pipe_i->i_pipe = kmalloc(sizeof(struct pipe_inode_info), KMALLOC_WAIT);
        pii = pipe_i->i_pipe;
        if (!pii) {
                set_errno(ENOMEM);
                goto error_kmalloc;
        }
        pii->p_buf = kpage_zalloc_addr();
        if (!pii->p_buf) {
                set_errno(ENOMEM);
                goto error_kpage;
        }
        pii->p_rd_off = 0;
        pii->p_wr_off = 0;
        pii->p_nr_readers = 0;
        pii->p_nr_writers = 0;
        cv_init(&pii->p_cv);    /* must do this before dentry_open / pipe_open */
        /* Now we have an inode for the pipe.  We need two files for the read and
         * write ends of the pipe. */
        flags &= ~(O_ACCMODE);  /* avoid user bugs */
        pipe_f_read = dentry_open(pipe_d, flags | O_RDONLY);
        if (!pipe_f_read)
                goto error_f_read;
        pipe_f_write = dentry_open(pipe_d, flags | O_WRONLY);
        if (!pipe_f_write)
                goto error_f_write;
        pipe_files[0] = pipe_f_read;
        pipe_files[1] = pipe_f_write;
        return 0;

error_f_write:
        kref_put(&pipe_f_read->f_kref);
error_f_read:
        page_decref(kva2page(pii->p_buf));
error_kpage:
        kfree(pipe_i->i_pipe);
error_kmalloc:
        /* We don't need to free the pipe_i; putting the dentry will free it */
error_post_dentry:
        /* Note we only free the dentry on failure. */
        kref_put(&pipe_d->d_kref);
        return -1;
}

struct file *alloc_file(void)
{
        struct file *file = kmem_cache_alloc(file_kcache, 0);
        if (!file) {
                set_errno(ENOMEM);
                return 0;
        }
        /* one for the ref passed out*/
        kref_init(&file->f_kref, file_release, 1);
        return file;
}

/* Opens and returns the file specified by dentry */
struct file *dentry_open(struct dentry *dentry, int flags)
{
        struct inode *inode;
        struct file *file;
        int desired_mode;
        inode = dentry->d_inode;
        /* Do the mode first, since we can still error out.  f_mode stores how the
         * OS file is open, which can be more restrictive than the i_mode */
        switch (flags & (O_RDONLY | O_WRONLY | O_RDWR)) {
                case O_RDONLY:
                        desired_mode = S_IRUSR;
                        break;
                case O_WRONLY:
                        desired_mode = S_IWUSR;
                        break;
                case O_RDWR:
                        desired_mode = S_IRUSR | S_IWUSR;
                        break;
                default:
                        goto error_access;
        }
        if (check_perms(inode, desired_mode))
                goto error_access;
        file = alloc_file();
        if (!file)
                return 0;
        file->f_mode = desired_mode;
        /* Add to the list of all files of this SB */
        TAILQ_INSERT_TAIL(&inode->i_sb->s_files, file, f_list);
        kref_get(&dentry->d_kref, 1);
        file->f_dentry = dentry;
        kref_get(&inode->i_sb->s_mount->mnt_kref, 1);
        file->f_vfsmnt = inode->i_sb->s_mount;          /* saving a ref to the vmnt...*/
        file->f_op = inode->i_fop;
        /* Don't store open mode or creation flags */
        file->f_flags = flags & ~(O_ACCMODE | O_CREAT_FLAGS);
        file->f_pos = 0;
        file->f_uid = inode->i_uid;
        file->f_gid = inode->i_gid;
        file->f_error = 0;
//      struct event_poll_tailq         f_ep_links;
        spinlock_init(&file->f_ep_lock);
        file->f_privdata = 0;                                           /* prob overriden by the fs */
        file->f_mapping = inode->i_mapping;
        file->f_op->open(inode, file);
        return file;
error_access:
        set_errno(EACCES);
        return 0;
}

/* Closes a file, fsync, whatever else is necessary.  Called when the kref hits
 * 0.  Note that the file is not refcounted on the s_files list, nor is the
 * f_mapping refcounted (it is pinned by the i_mapping). */
void file_release(struct kref *kref)
{
        struct file *file = container_of(kref, struct file, f_kref);

        struct super_block *sb = file->f_dentry->d_sb;
        spin_lock(&sb->s_lock);
        TAILQ_REMOVE(&sb->s_files, file, f_list);
        spin_unlock(&sb->s_lock);

        /* TODO: fsync (BLK).  also, we may want to parallelize the blocking that
         * could happen in here (spawn kernel threads)... */
        file->f_op->release(file->f_dentry->d_inode, file);
        /* Clean up the other refs we hold */
        kref_put(&file->f_dentry->d_kref);
        kref_put(&file->f_vfsmnt->mnt_kref);
        kmem_cache_free(file_kcache, file);
}

/* Process-related File management functions */

/* Given any FD, get the appropriate file, 0 o/w */
struct file *get_file_from_fd(struct files_struct *open_files, int file_desc)
{
        struct file *retval = 0;
        if (file_desc < 0)
                return 0;
        spin_lock(&open_files->lock);
        if (open_files->closed) {
                spin_unlock(&open_files->lock);
                return 0;
        }
        if (file_desc < open_files->max_fdset) {
                if (GET_BITMASK_BIT(open_files->open_fds->fds_bits, file_desc)) {
                        /* while max_files and max_fdset might not line up, we should never
                         * have a valid fdset higher than files */
                        assert(file_desc < open_files->max_files);
                        retval = open_files->fd[file_desc].fd_file;
                        /* 9ns might be using this one, in which case file == 0 */
                        if (retval)
                                kref_get(&retval->f_kref, 1);
                }
        }
        spin_unlock(&open_files->lock);
        return retval;
}

/* 9ns: puts back an FD from the VFS-FD-space. */
int put_fd(struct files_struct *open_files, int file_desc)
{
        if (file_desc < 0) {
                warn("Negative FD!\n");
                return 0;
        }
        spin_lock(&open_files->lock);
        if (file_desc < open_files->max_fdset) {
                if (GET_BITMASK_BIT(open_files->open_fds->fds_bits, file_desc)) {
                        /* while max_files and max_fdset might not line up, we should never
                         * have a valid fdset higher than files */
                        assert(file_desc < open_files->max_files);
                        CLR_BITMASK_BIT(open_files->open_fds->fds_bits, file_desc);
                }
        }
        spin_unlock(&open_files->lock);
        return 0;
}

/* Remove FD from the open files, if it was there, and return f.  Currently,
 * this decref's f, so the return value is not consumable or even usable.  This
 * hasn't been thought through yet. */
struct file *put_file_from_fd(struct files_struct *open_files, int file_desc)
{
        struct file *file = 0;
        if (file_desc < 0)
                return 0;
        spin_lock(&open_files->lock);
        if (file_desc < open_files->max_fdset) {
                if (GET_BITMASK_BIT(open_files->open_fds->fds_bits, file_desc)) {
                        /* while max_files and max_fdset might not line up, we should never
                         * have a valid fdset higher than files */
                        assert(file_desc < open_files->max_files);
                        file = open_files->fd[file_desc].fd_file;
                        open_files->fd[file_desc].fd_file = 0;
                        assert(file);
                        kref_put(&file->f_kref);
                        CLR_BITMASK_BIT(open_files->open_fds->fds_bits, file_desc);
                }
        }
        spin_unlock(&open_files->lock);
        return file;
}

static int __get_fd(struct files_struct *open_files, int low_fd)
{
        int slot = -1;
        if ((low_fd < 0) || (low_fd > NR_FILE_DESC_MAX))
                return -EINVAL;
        if (open_files->closed)
                return -EINVAL; /* won't matter, they are dying */
        for (int i = low_fd; i < open_files->max_fdset; i++) {
                if (GET_BITMASK_BIT(open_files->open_fds->fds_bits, i))
                        continue;
                slot = i;
                SET_BITMASK_BIT(open_files->open_fds->fds_bits, slot);
                assert(slot < open_files->max_files &&
                       open_files->fd[slot].fd_file == 0);
                if (slot >= open_files->next_fd)
                        open_files->next_fd = slot + 1;
                break;
        }
        if (slot == -1) /* should expand the FD array and fd_set */
                warn("Ran out of file descriptors, deal with me!");
        return slot;
}

/* Gets and claims a free FD, used by 9ns.  < 0 == error. */
int get_fd(struct files_struct *open_files, int low_fd)
{
        int slot;
        spin_lock(&open_files->lock);
        slot = __get_fd(open_files, low_fd);
        spin_unlock(&open_files->lock);
        return slot;
}

/* Inserts the file in the files_struct, returning the corresponding new file
 * descriptor, or an error code.  We start looking for open fds from low_fd. */
int insert_file(struct files_struct *open_files, struct file *file, int low_fd)
{
        int slot;
        spin_lock(&open_files->lock);
        slot = __get_fd(open_files, low_fd);
        if (slot < 0) {
                spin_unlock(&open_files->lock);
                return slot;
        }
        assert(slot < open_files->max_files &&
               open_files->fd[slot].fd_file == 0);
        kref_get(&file->f_kref, 1);
        open_files->fd[slot].fd_file = file;
        open_files->fd[slot].fd_flags = 0;
        spin_unlock(&open_files->lock);
        return slot;
}

/* Closes all open files.  Mostly just a "put" for all files.  If cloexec, it
 * will only close files that are opened with O_CLOEXEC. */
void close_all_files(struct files_struct *open_files, bool cloexec)
{
        struct file *file;
        spin_lock(&open_files->lock);
        if (open_files->closed) {
                spin_unlock(&open_files->lock);
                return;
        }
        if (!cloexec)
                open_files->closed = TRUE;
        for (int i = 0; i < open_files->max_fdset; i++) {
                if (GET_BITMASK_BIT(open_files->open_fds->fds_bits, i)) {
                        /* while max_files and max_fdset might not line up, we should never
                         * have a valid fdset higher than files */
                        assert(i < open_files->max_files);
                        file = open_files->fd[i].fd_file;
                        /* no file == 9ns uses the FD.  they will deal with it */
                        if (!file)
                                continue;
                        if (cloexec && !(open_files->fd[i].fd_flags & O_CLOEXEC))
                                continue;
                        /* Actually close the file */
                        open_files->fd[i].fd_file = 0;
                        assert(file);
                        kref_put(&file->f_kref);
                        CLR_BITMASK_BIT(open_files->open_fds->fds_bits, i);
                }
        }
        spin_unlock(&open_files->lock);
}

/* Inserts all of the files from src into dst, used by sys_fork(). */
void clone_files(struct files_struct *src, struct files_struct *dst)
{
        struct file *file;
        spin_lock(&src->lock);
        if (src->closed) {
                spin_unlock(&src->lock);
                return;
        }
        spin_lock(&dst->lock);
        if (dst->closed) {
                warn("Destination closed before it opened");
                spin_unlock(&dst->lock);
                spin_unlock(&src->lock);
                return;
        }
        for (int i = 0; i < src->max_fdset; i++) {
                if (GET_BITMASK_BIT(src->open_fds->fds_bits, i)) {
                        /* while max_files and max_fdset might not line up, we should never
                         * have a valid fdset higher than files */
                        assert(i < src->max_files);
                        file = src->fd[i].fd_file;
                        assert(i < dst->max_files && dst->fd[i].fd_file == 0);
                        SET_BITMASK_BIT(dst->open_fds->fds_bits, i);
                        dst->fd[i].fd_file = file;
                        assert(file);
                        kref_get(&file->f_kref, 1);
                        if (i >= dst->next_fd)
                                dst->next_fd = i + 1;
                }
        }
        spin_unlock(&dst->lock);
        spin_unlock(&src->lock);
}

/* Change the working directory of the given fs env (one per process, at this
 * point).  Returns 0 for success, -ERROR for whatever error. */
int do_chdir(struct fs_struct *fs_env, char *path)
{
        struct nameidata nd_r = {0}, *nd = &nd_r;
        int retval;
        retval = path_lookup(path, LOOKUP_DIRECTORY, nd);
        if (!retval) {
                /* nd->dentry is the place we want our PWD to be */
                kref_get(&nd->dentry->d_kref, 1);
                kref_put(&fs_env->pwd->d_kref);
                fs_env->pwd = nd->dentry;
        }
        path_release(nd);
        return retval;
}

/* Returns a null-terminated string of up to length cwd_l containing the
 * absolute path of fs_env, (up to fs_env's root).  Be sure to kfree the char*
 * "kfree_this" when you are done with it.  We do this since it's easier to
 * build this string going backwards.  Note cwd_l is not a strlen, it's an
 * absolute size. */
char *do_getcwd(struct fs_struct *fs_env, char **kfree_this, size_t cwd_l)
{
        struct dentry *dentry = fs_env->pwd;
        size_t link_len;
        char *path_start, *kbuf;

        if (cwd_l < 2) {
                set_errno(ERANGE);
                return 0;
        }
        kbuf = kmalloc(cwd_l, 0);
        if (!kbuf) {
                set_errno(ENOMEM);
                return 0;
        }
        *kfree_this = kbuf;
        kbuf[cwd_l - 1] = '\0';
        kbuf[cwd_l - 2] = '/';
        /* for each dentry in the path, all the way back to the root of fs_env, we
         * grab the dentry name, push path_start back enough, and write in the name,
         * using /'s to terminate.  We skip the root, since we don't want it's
         * actual name, just "/", which is set before each loop. */
        path_start = kbuf + cwd_l - 2;  /* the last byte written */
        while (dentry != fs_env->root) {
                link_len = dentry->d_name.len;          /* this does not count the \0 */
                if (path_start - (link_len + 2) < kbuf) {
                        kfree(kbuf);
                        set_errno(ERANGE);
                        return 0;
                }
                path_start -= link_len + 1;     /* the 1 is for the \0 */
                strncpy(path_start, dentry->d_name.name, link_len);
                path_start--;
                *path_start = '/';
                dentry = dentry->d_parent;      
        }
        return path_start;
}

static void print_dir(struct dentry *dentry, char *buf, int depth)
{
        struct dentry *child_d;
        struct dirent next = {0};
        struct file *dir;
        int retval;

        if (!S_ISDIR(dentry->d_inode->i_mode)) {
                warn("Thought this was only directories!!");
                return;
        }
        /* Print this dentry */
        printk("%s%s/ nlink: %d\n", buf, dentry->d_name.name,
               dentry->d_inode->i_nlink);
        if (dentry->d_mount_point) {
                dentry = dentry->d_mounted_fs->mnt_root;
        }
        if (depth >= 32)
                return;
        /* Set buffer for our kids */
        buf[depth] = '\t';
        dir = dentry_open(dentry, 0);
        if (!dir)
                panic("Filesystem seems inconsistent - unable to open a dir!");
        /* Process every child, recursing on directories */
        while (1) {
                retval = dir->f_op->readdir(dir, &next);
                if (retval >= 0) {
                        /* Skip .., ., and empty entries */
                        if (!strcmp("..", next.d_name) || !strcmp(".", next.d_name) ||
                            next.d_ino == 0)
                                goto loop_next;
                        /* there is an entry, now get its dentry */
                        child_d = do_lookup(dentry, next.d_name);
                        if (!child_d)
                                panic("Inconsistent FS, dirent doesn't have a dentry!");
                        /* Recurse for directories, or just print the name for others */
                        switch (child_d->d_inode->i_mode & __S_IFMT) {
                                case (__S_IFDIR):
                                        print_dir(child_d, buf, depth + 1);
                                        break;
                                case (__S_IFREG):
                                        printk("%s%s size(B): %d nlink: %d\n", buf, next.d_name,
                                               child_d->d_inode->i_size, child_d->d_inode->i_nlink);
                                        break;
                                case (__S_IFLNK):
                                        printk("%s%s -> %s\n", buf, next.d_name,
                                               child_d->d_inode->i_op->readlink(child_d));
                                        break;
                                case (__S_IFCHR):
                                        printk("%s%s (char device) nlink: %d\n", buf, next.d_name,
                                               child_d->d_inode->i_nlink);
                                        break;
                                case (__S_IFBLK):
                                        printk("%s%s (block device) nlink: %d\n", buf, next.d_name,
                                               child_d->d_inode->i_nlink);
                                        break;
                                default:
                                        warn("Look around you!  Unknown filetype!");
                        }
                        kref_put(&child_d->d_kref);     
                }
loop_next:
                if (retval <= 0)
                        break;
        }
        /* Reset buffer to the way it was */
        buf[depth] = '\0';
        kref_put(&dir->f_kref);
}

/* Debugging */
int ls_dash_r(char *path)
{
        struct nameidata nd_r = {0}, *nd = &nd_r;
        int error;
        char buf[32] = {0};

        error = path_lookup(path, LOOKUP_ACCESS | LOOKUP_DIRECTORY, nd);
        if (error) {
                path_release(nd);
                return error;
        }
        print_dir(nd->dentry, buf, 0);
        path_release(nd);
        return 0;
}

/* Dummy ops, to catch weird operations we weren't expecting */
int dummy_create(struct inode *dir, struct dentry *dentry, int mode,
                 struct nameidata *nd)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

struct dentry *dummy_lookup(struct inode *dir, struct dentry *dentry,
                          struct nameidata *nd)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return 0;
}

int dummy_link(struct dentry *old_dentry, struct inode *dir,
             struct dentry *new_dentry)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

int dummy_unlink(struct inode *dir, struct dentry *dentry)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

int dummy_symlink(struct inode *dir, struct dentry *dentry, const char *symname)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

int dummy_mkdir(struct inode *dir, struct dentry *dentry, int mode)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

int dummy_rmdir(struct inode *dir, struct dentry *dentry)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

int dummy_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t rdev)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

int dummy_rename(struct inode *old_dir, struct dentry *old_dentry,
               struct inode *new_dir, struct dentry *new_dentry)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

char *dummy_readlink(struct dentry *dentry)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return 0;
}

void dummy_truncate(struct inode *inode)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
}

int dummy_permission(struct inode *inode, int mode, struct nameidata *nd)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

int dummy_d_revalidate(struct dentry *dir, struct nameidata *nd)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

int dummy_d_hash(struct dentry *dentry, struct qstr *name)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

int dummy_d_compare(struct dentry *dir, struct qstr *name1, struct qstr *name2)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

int dummy_d_delete(struct dentry *dentry)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

int dummy_d_release(struct dentry *dentry)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
        return -1;
}

void dummy_d_iput(struct dentry *dentry, struct inode *inode)
{
        printk("Dummy VFS function %s called!\n", __FUNCTION__);
}

struct inode_operations dummy_i_op = {
        dummy_create,
        dummy_lookup,
        dummy_link,
        dummy_unlink,
        dummy_symlink,
        dummy_mkdir,
        dummy_rmdir,
        dummy_mknod,
        dummy_rename,
        dummy_readlink,
        dummy_truncate,
        dummy_permission,
};

struct dentry_operations dummy_d_op = {
        dummy_d_revalidate,
        dummy_d_hash,
        dummy_d_compare,
        dummy_d_delete,
        dummy_d_release,
        dummy_d_iput,
};