Account for .zfs ctldir inodes

[mirror_zfs-debian.git] / module / zfs / zfs_ctldir.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 *
 * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (C) 2011 Lawrence Livermore National Security, LLC.
 * Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER).
 * LLNL-CODE-403049.
 * Rewritten for Linux by:
 *   Rohan Puri <rohan.puri15@gmail.com>
 *   Brian Behlendorf <behlendorf1@llnl.gov>
 */

/*
 * ZFS control directory (a.k.a. ".zfs")
 *
 * This directory provides a common location for all ZFS meta-objects.
 * Currently, this is only the 'snapshot' and 'shares' directory, but this may
 * expand in the future.  The elements are built dynamically, as the hierarchy
 * does not actually exist on disk.
 *
 * For 'snapshot', we don't want to have all snapshots always mounted, because
 * this would take up a huge amount of space in /etc/mnttab.  We have three
 * types of objects:
 *
 *      ctldir ------> snapshotdir -------> snapshot
 *                                             |
 *                                             |
 *                                             V
 *                                         mounted fs
 *
 * The 'snapshot' node contains just enough information to lookup '..' and act
 * as a mountpoint for the snapshot.  Whenever we lookup a specific snapshot, we
 * perform an automount of the underlying filesystem and return the
 * corresponding inode.
 *
 * All mounts are handled automatically by an user mode helper which invokes
 * the mount mount procedure.  Unmounts are handled by allowing the mount
 * point to expire so the kernel may automatically unmount it.
 *
 * The '.zfs', '.zfs/snapshot', and all directories created under
 * '.zfs/snapshot' (ie: '.zfs/snapshot/<snapname>') all share the same
 * share the same zfs_sb_t as the head filesystem (what '.zfs' lives under).
 *
 * File systems mounted on top of the '.zfs/snapshot/<snapname>' paths
 * (ie: snapshots) are complete ZFS filesystems and have their own unique
 * zfs_sb_t.  However, the fsid reported by these mounts will be the same
 * as that used by the parent zfs_sb_t to make NFS happy.
 */

#include <sys/types.h>
#include <sys/param.h>
#include <sys/time.h>
#include <sys/systm.h>
#include <sys/sysmacros.h>
#include <sys/pathname.h>
#include <sys/vfs.h>
#include <sys/vfs_opreg.h>
#include <sys/zfs_ctldir.h>
#include <sys/zfs_ioctl.h>
#include <sys/zfs_vfsops.h>
#include <sys/zfs_vnops.h>
#include <sys/stat.h>
#include <sys/dmu.h>
#include <sys/dsl_deleg.h>
#include <sys/mount.h>
#include <sys/zpl.h>
#include "zfs_namecheck.h"

/*
 * Control Directory Tunables (.zfs)
 */
int zfs_expire_snapshot = ZFSCTL_EXPIRE_SNAPSHOT;

static zfs_snapentry_t *
zfsctl_sep_alloc(void)
{
        return kmem_zalloc(sizeof (zfs_snapentry_t), KM_SLEEP);
}

void
zfsctl_sep_free(zfs_snapentry_t *sep)
{
        kmem_free(sep->se_name, MAXNAMELEN);
        kmem_free(sep->se_path, PATH_MAX);
        kmem_free(sep, sizeof (zfs_snapentry_t));
}

/*
 * Attempt to expire an automounted snapshot, unmounts are attempted every
 * 'zfs_expire_snapshot' seconds until they succeed.  The work request is
 * responsible for rescheduling itself and freeing the zfs_expire_snapshot_t.
 */
static void
zfsctl_expire_snapshot(void *data)
{
        zfs_snapentry_t *sep;
        zfs_sb_t *zsb;
        int error;

        sep = spl_get_work_data(data, zfs_snapentry_t, se_work.work);
        zsb = ITOZSB(sep->se_inode);

        error = zfsctl_unmount_snapshot(zsb, sep->se_name, MNT_EXPIRE);
        if (error == EBUSY)
                schedule_delayed_work(&sep->se_work, zfs_expire_snapshot * HZ);
}

int
snapentry_compare(const void *a, const void *b)
{
        const zfs_snapentry_t *sa = a;
        const zfs_snapentry_t *sb = b;
        int ret = strcmp(sa->se_name, sb->se_name);

        if (ret < 0)
                return (-1);
        else if (ret > 0)
                return (1);
        else
                return (0);
}

boolean_t
zfsctl_is_node(struct inode *ip)
{
        return (ITOZ(ip)->z_is_ctldir);
}

boolean_t
zfsctl_is_snapdir(struct inode *ip)
{
        return (zfsctl_is_node(ip) && (ip->i_ino <= ZFSCTL_INO_SNAPDIRS));
}

/*
 * Allocate a new inode with the passed id and ops.
 */
static struct inode *
zfsctl_inode_alloc(zfs_sb_t *zsb, uint64_t id,
    const struct file_operations *fops, const struct inode_operations *ops)
{
        struct timespec now = current_fs_time(zsb->z_sb);
        struct inode *ip;
        znode_t *zp;

        ip = new_inode(zsb->z_sb);
        if (ip == NULL)
                return (NULL);

        zp = ITOZ(ip);
        ASSERT3P(zp->z_dirlocks, ==, NULL);
        ASSERT3P(zp->z_acl_cached, ==, NULL);
        ASSERT3P(zp->z_xattr_cached, ==, NULL);
        zp->z_id = id;
        zp->z_unlinked = 0;
        zp->z_atime_dirty = 0;
        zp->z_zn_prefetch = 0;
        zp->z_moved = 0;
        zp->z_sa_hdl = NULL;
        zp->z_blksz = 0;
        zp->z_seq = 0;
        zp->z_mapcnt = 0;
        zp->z_gen = 0;
        zp->z_size = 0;
        zp->z_atime[0] = 0;
        zp->z_atime[1] = 0;
        zp->z_links = 0;
        zp->z_pflags = 0;
        zp->z_uid = 0;
        zp->z_gid = 0;
        zp->z_mode = 0;
        zp->z_sync_cnt = 0;
        zp->z_is_zvol = B_FALSE;
        zp->z_is_mapped = B_FALSE;
        zp->z_is_ctldir = B_TRUE;
        zp->z_is_sa = B_FALSE;
        ip->i_ino = id;
        ip->i_mode = (S_IFDIR | S_IRUGO | S_IXUGO);
        ip->i_uid = 0;
        ip->i_gid = 0;
        ip->i_blkbits = SPA_MINBLOCKSHIFT;
        ip->i_atime = now;
        ip->i_mtime = now;
        ip->i_ctime = now;
        ip->i_fop = fops;
        ip->i_op = ops;

        if (insert_inode_locked(ip)) {
                unlock_new_inode(ip);
                iput(ip);
                return (NULL);
        }

        mutex_enter(&zsb->z_znodes_lock);
        list_insert_tail(&zsb->z_all_znodes, zp);
        zsb->z_nr_znodes++;
        membar_producer();
        mutex_exit(&zsb->z_znodes_lock);

        unlock_new_inode(ip);

        return (ip);
}

/*
 * Lookup the inode with given id, it will be allocated if needed.
 */
static struct inode *
zfsctl_inode_lookup(zfs_sb_t *zsb, unsigned long id,
    const struct file_operations *fops, const struct inode_operations *ops)
{
        struct inode *ip = NULL;

        while (ip == NULL) {
                ip = ilookup(zsb->z_sb, id);
                if (ip)
                        break;

                /* May fail due to concurrent zfsctl_inode_alloc() */
                ip = zfsctl_inode_alloc(zsb, id, fops, ops);
        }

        return (ip);
}

/*
 * Free zfsctl inode specific structures, currently there are none.
 */
void
zfsctl_inode_destroy(struct inode *ip)
{
        return;
}

/*
 * An inode is being evicted from the cache.
 */
void
zfsctl_inode_inactive(struct inode *ip)
{
        if (zfsctl_is_snapdir(ip))
                zfsctl_snapdir_inactive(ip);
}

/*
 * Create the '.zfs' directory.  This directory is cached as part of the VFS
 * structure.  This results in a hold on the zfs_sb_t.  The code in zfs_umount()
 * therefore checks against a vfs_count of 2 instead of 1.  This reference
 * is removed when the ctldir is destroyed in the unmount.  All other entities
 * under the '.zfs' directory are created dynamically as needed.
 */
int
zfsctl_create(zfs_sb_t *zsb)
{
        ASSERT(zsb->z_ctldir == NULL);

        zsb->z_ctldir = zfsctl_inode_alloc(zsb, ZFSCTL_INO_ROOT,
            &zpl_fops_root, &zpl_ops_root);
        if (zsb->z_ctldir == NULL)
                return (ENOENT);

        return (0);
}

/*
 * Destroy the '.zfs' directory.  Only called when the filesystem is unmounted.
 */
void
zfsctl_destroy(zfs_sb_t *zsb)
{
        iput(zsb->z_ctldir);
        zsb->z_ctldir = NULL;
}

/*
 * Given a root znode, retrieve the associated .zfs directory.
 * Add a hold to the vnode and return it.
 */
struct inode *
zfsctl_root(znode_t *zp)
{
        ASSERT(zfs_has_ctldir(zp));
        igrab(ZTOZSB(zp)->z_ctldir);
        return (ZTOZSB(zp)->z_ctldir);
}

/*ARGSUSED*/
int
zfsctl_fid(struct inode *ip, fid_t *fidp)
{
        znode_t         *zp = ITOZ(ip);
        zfs_sb_t        *zsb = ITOZSB(ip);
        uint64_t        object = zp->z_id;
        zfid_short_t    *zfid;
        int             i;

        ZFS_ENTER(zsb);

        if (fidp->fid_len < SHORT_FID_LEN) {
                fidp->fid_len = SHORT_FID_LEN;
                ZFS_EXIT(zsb);
                return (ENOSPC);
        }

        zfid = (zfid_short_t *)fidp;

        zfid->zf_len = SHORT_FID_LEN;

        for (i = 0; i < sizeof (zfid->zf_object); i++)
                zfid->zf_object[i] = (uint8_t)(object >> (8 * i));

        /* .zfs znodes always have a generation number of 0 */
        for (i = 0; i < sizeof (zfid->zf_gen); i++)
                zfid->zf_gen[i] = 0;

        ZFS_EXIT(zsb);
        return (0);
}

static int
zfsctl_snapshot_zname(struct inode *ip, const char *name, int len, char *zname)
{
        objset_t *os = ITOZSB(ip)->z_os;

        if (snapshot_namecheck(name, NULL, NULL) != 0)
                return (EILSEQ);

        dmu_objset_name(os, zname);
        if ((strlen(zname) + 1 + strlen(name)) >= len)
                return (ENAMETOOLONG);

        (void) strcat(zname, "@");
        (void) strcat(zname, name);

        return (0);
}

static int
zfsctl_snapshot_zpath(struct path *path, int len, char *zpath)
{
        char *path_buffer, *path_ptr;
        int path_len, error = 0;

        path_buffer = kmem_alloc(len, KM_SLEEP);

        path_ptr = d_path(path, path_buffer, len);
        if (IS_ERR(path_ptr)) {
                error = -PTR_ERR(path_ptr);
                goto out;
        }

        path_len = path_buffer + len - 1 - path_ptr;
        if (path_len > len) {
                error = EFAULT;
                goto out;
        }

        memcpy(zpath, path_ptr, path_len);
        zpath[path_len] = '\0';
out:
        kmem_free(path_buffer, len);

        return (error);
}

/*
 * Special case the handling of "..".
 */
/* ARGSUSED */
int
zfsctl_root_lookup(struct inode *dip, char *name, struct inode **ipp,
    int flags, cred_t *cr, int *direntflags, pathname_t *realpnp)
{
        zfs_sb_t *zsb = ITOZSB(dip);
        int error = 0;

        ZFS_ENTER(zsb);

        if (strcmp(name, "..") == 0) {
                *ipp = dip->i_sb->s_root->d_inode;
        } else if (strcmp(name, ZFS_SNAPDIR_NAME) == 0) {
                *ipp = zfsctl_inode_lookup(zsb, ZFSCTL_INO_SNAPDIR,
                    &zpl_fops_snapdir, &zpl_ops_snapdir);
        } else if (strcmp(name, ZFS_SHAREDIR_NAME) == 0) {
                *ipp = zfsctl_inode_lookup(zsb, ZFSCTL_INO_SHARES,
                    &zpl_fops_shares, &zpl_ops_shares);
        } else {
                *ipp = NULL;
        }

        if (*ipp == NULL)
                error = ENOENT;

        ZFS_EXIT(zsb);

        return (error);
}

/*
 * Lookup entry point for the 'snapshot' directory.  Try to open the
 * snapshot if it exist, creating the pseudo filesystem inode as necessary.
 * Perform a mount of the associated dataset on top of the inode.
 */
/* ARGSUSED */
int
zfsctl_snapdir_lookup(struct inode *dip, char *name, struct inode **ipp,
    int flags, cred_t *cr, int *direntflags, pathname_t *realpnp)
{
        zfs_sb_t *zsb = ITOZSB(dip);
        uint64_t id;
        int error;

        ZFS_ENTER(zsb);

        error = dmu_snapshot_id(zsb->z_os, name, &id);
        if (error) {
                ZFS_EXIT(zsb);
                return (error);
        }

        *ipp = zfsctl_inode_lookup(zsb, ZFSCTL_INO_SNAPDIRS - id,
            &simple_dir_operations, &simple_dir_inode_operations);
        if (*ipp) {
#ifdef HAVE_AUTOMOUNT
                (*ipp)->i_flags |= S_AUTOMOUNT;
#endif /* HAVE_AUTOMOUNT */
        } else {
                error = ENOENT;
        }

        ZFS_EXIT(zsb);

        return (error);
}

static void
zfsctl_rename_snap(zfs_sb_t *zsb, zfs_snapentry_t *sep, const char *name)
{
        avl_index_t where;

        ASSERT(MUTEX_HELD(&zsb->z_ctldir_lock));
        ASSERT(sep != NULL);

        /*
         * Change the name in the AVL tree.
         */
        avl_remove(&zsb->z_ctldir_snaps, sep);
        (void) strcpy(sep->se_name, name);
        VERIFY(avl_find(&zsb->z_ctldir_snaps, sep, &where) == NULL);
        avl_insert(&zsb->z_ctldir_snaps, sep, where);
}

/*
 * Renaming a directory under '.zfs/snapshot' will automatically trigger
 * a rename of the snapshot to the new given name.  The rename is confined
 * to the '.zfs/snapshot' directory snapshots cannot be moved elsewhere.
 */
/*ARGSUSED*/
int
zfsctl_snapdir_rename(struct inode *sdip, char *sname,
    struct inode *tdip, char *tname, cred_t *cr, int flags)
{
        zfs_sb_t *zsb = ITOZSB(sdip);
        zfs_snapentry_t search, *sep;
        avl_index_t where;
        char *to, *from, *real;
        int error;

        ZFS_ENTER(zsb);

        to = kmem_alloc(MAXNAMELEN, KM_SLEEP);
        from = kmem_alloc(MAXNAMELEN, KM_SLEEP);
        real = kmem_alloc(MAXNAMELEN, KM_SLEEP);

        if (zsb->z_case == ZFS_CASE_INSENSITIVE) {
                error = dmu_snapshot_realname(zsb->z_os, sname, real,
                    MAXNAMELEN, NULL);
                if (error == 0) {
                        sname = real;
                } else if (error != ENOTSUP) {
                        goto out;
                }
        }

        error = zfsctl_snapshot_zname(sdip, sname, MAXNAMELEN, from);
        if (!error)
                error = zfsctl_snapshot_zname(tdip, tname, MAXNAMELEN, to);
        if (!error)
                error = zfs_secpolicy_rename_perms(from, to, cr);
        if (error)
                goto out;

        /*
         * Cannot move snapshots out of the snapdir.
         */
        if (sdip != tdip) {
                error = EINVAL;
                goto out;
        }

        /*
         * No-op when names are identical.
         */
        if (strcmp(sname, tname) == 0) {
                error = 0;
                goto out;
        }

        mutex_enter(&zsb->z_ctldir_lock);

        error = dmu_objset_rename(from, to, B_FALSE);
        if (error)
                goto out_unlock;

        search.se_name = (char *)sname;
        sep = avl_find(&zsb->z_ctldir_snaps, &search, &where);
        if (sep)
                zfsctl_rename_snap(zsb, sep, tname);

out_unlock:
        mutex_exit(&zsb->z_ctldir_lock);
out:
        kmem_free(from, MAXNAMELEN);
        kmem_free(to, MAXNAMELEN);
        kmem_free(real, MAXNAMELEN);

        ZFS_EXIT(zsb);

        return (error);
}

/*
 * Removing a directory under '.zfs/snapshot' will automatically trigger
 * the removal of the snapshot with the given name.
 */
/* ARGSUSED */
int
zfsctl_snapdir_remove(struct inode *dip, char *name, cred_t *cr, int flags)
{
        zfs_sb_t *zsb = ITOZSB(dip);
        char *snapname, *real;
        int error;

        ZFS_ENTER(zsb);

        snapname = kmem_alloc(MAXNAMELEN, KM_SLEEP);
        real = kmem_alloc(MAXNAMELEN, KM_SLEEP);

        if (zsb->z_case == ZFS_CASE_INSENSITIVE) {
                error = dmu_snapshot_realname(zsb->z_os, name, real,
                    MAXNAMELEN, NULL);
                if (error == 0) {
                        name = real;
                } else if (error != ENOTSUP) {
                        goto out;
                }
        }

        error = zfsctl_snapshot_zname(dip, name, MAXNAMELEN, snapname);
        if (!error)
                error = zfs_secpolicy_destroy_perms(snapname, cr);
        if (error)
                goto out;

        error = zfsctl_unmount_snapshot(zsb, name, MNT_FORCE);
        if ((error == 0) || (error == ENOENT))
                error = dmu_objset_destroy(snapname, B_FALSE);
out:
        kmem_free(snapname, MAXNAMELEN);
        kmem_free(real, MAXNAMELEN);

        ZFS_EXIT(zsb);

        return (error);
}

/*
 * Creating a directory under '.zfs/snapshot' will automatically trigger
 * the creation of a new snapshot with the given name.
 */
/* ARGSUSED */
int
zfsctl_snapdir_mkdir(struct inode *dip, char *dirname, vattr_t *vap,
        struct inode **ipp, cred_t *cr, int flags)
{
        zfs_sb_t *zsb = ITOZSB(dip);
        char *dsname;
        int error;

        dsname = kmem_alloc(MAXNAMELEN, KM_SLEEP);

        if (snapshot_namecheck(dirname, NULL, NULL) != 0) {
                error = EILSEQ;
                goto out;
        }

        dmu_objset_name(zsb->z_os, dsname);

        error = zfs_secpolicy_snapshot_perms(dsname, cr);
        if (error)
                goto out;

        if (error == 0) {
                error = dmu_objset_snapshot(dsname, dirname,
                    NULL, NULL, B_FALSE, B_FALSE, -1);
                if (error)
                        goto out;

                error = zfsctl_snapdir_lookup(dip, dirname, ipp,
                    0, cr, NULL, NULL);
        }
out:
        kmem_free(dsname, MAXNAMELEN);

        return (error);
}

/*
 * When a .zfs/snapshot/<snapshot> inode is evicted they must be removed
 * from the snapshot list.  This will normally happen as part of the auto
 * unmount, however in the case of a manual snapshot unmount this will be
 * the only notification we receive.
 */
void
zfsctl_snapdir_inactive(struct inode *ip)
{
        zfs_sb_t *zsb = ITOZSB(ip);
        zfs_snapentry_t *sep, *next;

        mutex_enter(&zsb->z_ctldir_lock);

        sep = avl_first(&zsb->z_ctldir_snaps);
        while (sep != NULL) {
                next = AVL_NEXT(&zsb->z_ctldir_snaps, sep);

                if (sep->se_inode == ip) {
                        avl_remove(&zsb->z_ctldir_snaps, sep);
                        cancel_delayed_work_sync(&sep->se_work);
                        zfsctl_sep_free(sep);
                        break;
                }
                sep = next;
        }

        mutex_exit(&zsb->z_ctldir_lock);
}

/*
 * Attempt to unmount a snapshot by making a call to user space.
 * There is no assurance that this can or will succeed, is just a
 * best effort.  In the case where it does fail, perhaps because
 * it's in use, the unmount will fail harmlessly.
 */
#define SET_UNMOUNT_CMD \
        "exec 0</dev/null " \
        "     1>/dev/null " \
        "     2>/dev/null; " \
        "umount -t zfs -n %s%s"

static int
__zfsctl_unmount_snapshot(zfs_snapentry_t *sep, int flags)
{
        char *argv[] = { "/bin/sh", "-c", NULL, NULL };
        char *envp[] = { NULL };
        int error;

        argv[2] = kmem_asprintf(SET_UNMOUNT_CMD,
            flags & MNT_FORCE ? "-f " : "", sep->se_path);
        error = call_usermodehelper(argv[0], argv, envp, 1);
        strfree(argv[2]);

        /*
         * The umount system utility will return 256 on error.  We must
         * assume this error is because the file system is busy so it is
         * converted to the more sensible EBUSY.
         */
        if (error)
                error = EBUSY;

        /*
         * This was the result of a manual unmount, cancel the delayed work
         * to prevent zfsctl_expire_snapshot() from attempting a unmount.
         */
        if ((error == 0) && !(flags & MNT_EXPIRE))
                cancel_delayed_work(&sep->se_work);

        return (error);
}

int
zfsctl_unmount_snapshot(zfs_sb_t *zsb, char *name, int flags)
{
        zfs_snapentry_t search;
        zfs_snapentry_t *sep;
        int error = 0;

        mutex_enter(&zsb->z_ctldir_lock);

        search.se_name = name;
        sep = avl_find(&zsb->z_ctldir_snaps, &search, NULL);
        if (sep) {
                avl_remove(&zsb->z_ctldir_snaps, sep);
                error = __zfsctl_unmount_snapshot(sep, flags);
                if (error == EBUSY)
                        avl_add(&zsb->z_ctldir_snaps, sep);
                else
                        zfsctl_sep_free(sep);
        } else {
                error = ENOENT;
        }

        mutex_exit(&zsb->z_ctldir_lock);
        ASSERT3S(error, >=, 0);

        return (error);
}

/*
 * Traverse all mounted snapshots and attempt to unmount them.  This
 * is best effort, on failure EEXIST is returned and count will be set
 * to the number of file snapshots which could not be unmounted.
 */
int
zfsctl_unmount_snapshots(zfs_sb_t *zsb, int flags, int *count)
{
        zfs_snapentry_t *sep, *next;
        int error = 0;

        *count = 0;

        ASSERT(zsb->z_ctldir != NULL);
        mutex_enter(&zsb->z_ctldir_lock);

        sep = avl_first(&zsb->z_ctldir_snaps);
        while (sep != NULL) {
                next = AVL_NEXT(&zsb->z_ctldir_snaps, sep);
                avl_remove(&zsb->z_ctldir_snaps, sep);
                error = __zfsctl_unmount_snapshot(sep, flags);
                if (error == EBUSY) {
                        avl_add(&zsb->z_ctldir_snaps, sep);
                        (*count)++;
                } else {
                        zfsctl_sep_free(sep);
                }

                sep = next;
        }

        mutex_exit(&zsb->z_ctldir_lock);

        return ((*count > 0) ? EEXIST : 0);
}

#define SET_MOUNT_CMD \
        "exec 0</dev/null " \
        "     1>/dev/null " \
        "     2>/dev/null; " \
        "mount -t zfs -n %s %s"

int
zfsctl_mount_snapshot(struct path *path, int flags)
{
        struct dentry *dentry = path->dentry;
        struct inode *ip = dentry->d_inode;
        zfs_sb_t *zsb = ITOZSB(ip);
        char *full_name, *full_path;
        zfs_snapentry_t *sep;
        zfs_snapentry_t search;
        char *argv[] = { "/bin/sh", "-c", NULL, NULL };
        char *envp[] = { NULL };
        int error;

        ZFS_ENTER(zsb);

        full_name = kmem_zalloc(MAXNAMELEN, KM_SLEEP);
        full_path = kmem_zalloc(PATH_MAX, KM_SLEEP);

        error = zfsctl_snapshot_zname(ip, dname(dentry), MAXNAMELEN, full_name);
        if (error)
                goto error;

        error = zfsctl_snapshot_zpath(path, PATH_MAX, full_path);
        if (error)
                goto error;

        /*
         * Attempt to mount the snapshot from user space.  Normally this
         * would be done using the vfs_kern_mount() function, however that
         * function is marked GPL-only and cannot be used.  On error we
         * careful to log the real error to the console and return EISDIR
         * to safely abort the automount.  This should be very rare.
         */
        argv[2] = kmem_asprintf(SET_MOUNT_CMD, full_name, full_path);
        error = call_usermodehelper(argv[0], argv, envp, 1);
        strfree(argv[2]);
        if (error) {
                printk("ZFS: Unable to automount %s at %s: %d\n",
                    full_name, full_path, error);
                error = EISDIR;
                goto error;
        }

        mutex_enter(&zsb->z_ctldir_lock);

        /*
         * Ensure a previous entry does not exist, if it does safely remove
         * it any cancel the outstanding expiration.  This can occur when a
         * snapshot is manually unmounted and then an automount is triggered.
         */
        search.se_name = full_name;
        sep = avl_find(&zsb->z_ctldir_snaps, &search, NULL);
        if (sep) {
                avl_remove(&zsb->z_ctldir_snaps, sep);
                cancel_delayed_work_sync(&sep->se_work);
                zfsctl_sep_free(sep);
        }

        sep = zfsctl_sep_alloc();
        sep->se_name = full_name;
        sep->se_path = full_path;
        sep->se_inode = ip;
        avl_add(&zsb->z_ctldir_snaps, sep);

        spl_init_delayed_work(&sep->se_work, zfsctl_expire_snapshot, sep);
        schedule_delayed_work(&sep->se_work, zfs_expire_snapshot * HZ);

        mutex_exit(&zsb->z_ctldir_lock);
error:
        if (error) {
                kmem_free(full_name, MAXNAMELEN);
                kmem_free(full_path, PATH_MAX);
        }

        ZFS_EXIT(zsb);

        return (error);
}

/*
 * Check if this super block has a matching objset id.
 */
static int
zfsctl_test_super(struct super_block *sb, void *objsetidp)
{
        zfs_sb_t *zsb = sb->s_fs_info;
        uint64_t objsetid = *(uint64_t *)objsetidp;

        return (dmu_objset_id(zsb->z_os) == objsetid);
}

/*
 * Prevent a new super block from being allocated if an existing one
 * could not be located.  We only want to preform a lookup operation.
 */
static int
zfsctl_set_super(struct super_block *sb, void *objsetidp)
{
        return (-EEXIST);
}

int
zfsctl_lookup_objset(struct super_block *sb, uint64_t objsetid, zfs_sb_t **zsbp)
{
        zfs_sb_t *zsb = sb->s_fs_info;
        struct super_block *sbp;
        zfs_snapentry_t *sep;
        uint64_t id;
        int error;

        ASSERT(zsb->z_ctldir != NULL);

        mutex_enter(&zsb->z_ctldir_lock);

        /*
         * Verify that the snapshot is mounted.
         */
        sep = avl_first(&zsb->z_ctldir_snaps);
        while (sep != NULL) {
                error = dmu_snapshot_id(zsb->z_os, sep->se_name, &id);
                if (error)
                        goto out;

                if (id == objsetid)
                        break;

                sep = AVL_NEXT(&zsb->z_ctldir_snaps, sep);
        }

        if (sep != NULL) {
                /*
                 * Lookup the mounted root rather than the covered mount
                 * point.  This may fail if the snapshot has just been
                 * unmounted by an unrelated user space process.  This
                 * race cannot occur to an expired mount point because
                 * we hold the zsb->z_ctldir_lock to prevent the race.
                 */
                sbp = sget(&zpl_fs_type, zfsctl_test_super,
                    zfsctl_set_super, &id);
                if (IS_ERR(sbp)) {
                        error = -PTR_ERR(sbp);
                } else {
                        *zsbp = sbp->s_fs_info;
                        deactivate_super(sbp);
                }
        } else {
                error = EINVAL;
        }
out:
        mutex_exit(&zsb->z_ctldir_lock);
        ASSERT3S(error, >=, 0);

        return (error);
}

/* ARGSUSED */
int
zfsctl_shares_lookup(struct inode *dip, char *name, struct inode **ipp,
    int flags, cred_t *cr, int *direntflags, pathname_t *realpnp)
{
        zfs_sb_t *zsb = ITOZSB(dip);
        struct inode *ip;
        znode_t *dzp;
        int error;

        ZFS_ENTER(zsb);

        if (zsb->z_shares_dir == 0) {
                ZFS_EXIT(zsb);
                return (-ENOTSUP);
        }

        error = zfs_zget(zsb, zsb->z_shares_dir, &dzp);
        if (error) {
                ZFS_EXIT(zsb);
                return (error);
        }

        error = zfs_lookup(ZTOI(dzp), name, &ip, 0, cr, NULL, NULL);

        iput(ZTOI(dzp));
        ZFS_EXIT(zsb);

        return (error);
}


/*
 * Initialize the various pieces we'll need to create and manipulate .zfs
 * directories.  Currently this is unused but available.
 */
void
zfsctl_init(void)
{
}

/*
 * Cleanup the various pieces we needed for .zfs directories.  In particular
 * ensure the expiry timer is canceled safely.
 */
void
zfsctl_fini(void)
{
}

module_param(zfs_expire_snapshot, int, 0644);
MODULE_PARM_DESC(zfs_expire_snapshot, "Seconds to expire .zfs/snapshot");