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new helper: mount_subtree()

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takes vfsmount and relative path, does lookup within that vfsmount
(possibly triggering automounts) and returns the result as root
of subtree suitable for return by ->mount() (i.e. a reference to
dentry and an active reference to its superblock grabbed, superblock
locked exclusive).

btrfs and nfs switched to it instead of open-coding the sucker.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro
2011-11-16 21:43:59 -05:00
parent c133449587
commit ea441d1104
4 changed files with 41 additions and 53 deletions
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28
fs/namespace.c
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@ -2490,6 +2490,34 @@ struct mnt_namespace *create_mnt_ns(struct vfsmount *mnt)
}
EXPORT_SYMBOL(create_mnt_ns);
struct dentry *mount_subtree(struct vfsmount *mnt, const char *name)
{
struct mnt_namespace *ns;
struct path path;
int err;
ns = create_mnt_ns(mnt);
if (IS_ERR(ns))
return ERR_CAST(ns);
err = vfs_path_lookup(mnt->mnt_root, mnt,
name, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &path);
put_mnt_ns(ns);
if (err)
return ERR_PTR(err);
/* trade a vfsmount reference for active sb one */
atomic_inc(&path.mnt->mnt_sb->s_active);
mntput(path.mnt);
/* lock the sucker */
down_write(&path.mnt->mnt_sb->s_umount);
/* ... and return the root of (sub)tree on it */
return path.dentry;
}
EXPORT_SYMBOL(mount_subtree);
SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
char __user *, type, unsigned long, flags, void __user *, data)
{