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SubjectRe: [PATCH 21/38] union-mount: Support for mounting union mount file systems
On Tue, Jun 15, 2010 at 11:39:51AM -0700, Valerie Aurora wrote:
> Create and tear down union mount structures on mount. Check
> requirements for union mounts. This version clones the read-only
> mounts and puts them in an array hanging off the superblock of the
> topmost layer.
>
> XXX - need array? maybe use mnt_child or mnt_hash instead
>
> Thanks to Felix Fietkau <nbd@openwrt.org> for a bug fix.
> ---
> fs/namespace.c | 231 ++++++++++++++++++++++++++++++++++++++++++++++++-
> fs/super.c | 1 +
> include/linux/fs.h | 3 +
> include/linux/mount.h | 2 +
> 4 files changed, 235 insertions(+), 2 deletions(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 7a399ba..9f3884c 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -33,6 +33,7 @@
> #include <asm/unistd.h>
> #include "pnode.h"
> #include "internal.h"
> +#include "union.h"
>
> #define HASH_SHIFT ilog2(PAGE_SIZE / sizeof(struct list_head))
> #define HASH_SIZE (1UL << HASH_SHIFT)
> @@ -1049,6 +1050,7 @@ void umount_tree(struct vfsmount *mnt, int propagate, struct list_head *kill)
> propagate_umount(kill);
>
> list_for_each_entry(p, kill, mnt_hash) {
> + d_free_unions(p->mnt_root);
> list_del_init(&p->mnt_expire);
> list_del_init(&p->mnt_list);
> __touch_mnt_namespace(p->mnt_ns);
> @@ -1334,6 +1336,193 @@ static int invent_group_ids(struct vfsmount *mnt, bool recurse)
> return 0;
> }
>
> +/**
> + * check_mnt_union - mount-time checks for union mount
> + *
> + * @mntpnt: path of the mountpoint the new mount will be on
> + * @topmost_mnt: vfsmount of the new file system to be mounted
> + * @mnt_flags: mount flags for the new file system
> + *
> + * Mount-time check of upper and lower layer file systems to see if we
> + * can union mount one on the other.
> + *
> + * The rules:
> + *
> + * Lower layer(s) read-only: We can't deal with namespace changes in
> + * the lower layers of a union, so the lower layer must be read-only.
> + * Note that we could possibly convert a read-write unioned mount into
> + * a read-only mount here, which would give us a way to union more
> + * than one layer with separate mount commands.
> + *
> + * Union only at roots of file systems: Only permit unioning of file
> + * systems at their root directories. This allows us to mark entire
> + * mounts as unioned. Otherwise we must slowly and expensively work
> + * our way up a path looking for a unioned directory before we know if
> + * a path is from a unioned lower layer.
> + *
> + * No submounts. We could potentially mount over several read-only
> + * submounts, it's just more code to write.
> + *
> + * Topmost layer must be writable to support our readdir()
> + * solution of copying up all lower level entries to the
> + * topmost layer.
> + *
> + * Topmost file system must support whiteouts and fallthrus.
> + *
> + * Topmost file system can't be mounted elsewhere. XXX implement some
> + * kind of marker in the superblock so subsequent mounts are not
> + * possible.
> + *
> + * Note on union mounts and mount event propagation: The lower
> + * layer(s) of a union mount must not have any changes to its
> + * namespace. Therefore, it must not be part of any mount event
> + * propagation group - i.e., shared or slave. MNT_SHARED and
> + * MNT_SLAVE are not set at mount, but in do_change_type(), which
> + * prevents setting these flags on file systems with read-only users,
> + * which includes the lower layer(s) of a union mount.
> + */
> +
> +static int
> +check_mnt_union(struct path *mntpnt, struct vfsmount *topmost_mnt, int mnt_flags)
> +{
> + struct vfsmount *lower_mnt = mntpnt->mnt;
> +
> + if (!(mnt_flags & MNT_UNION))
> + return 0;
> +
> +#ifndef CONFIG_UNION_MOUNT
> + return -EINVAL;
> +#endif
> + if (!(lower_mnt->mnt_sb->s_flags & MS_RDONLY))
> + return -EBUSY;
> +
> + if (!list_empty(&lower_mnt->mnt_mounts))
> + return -EBUSY;
> +
> + if (!IS_ROOT(mntpnt->dentry))
> + return -EINVAL;
> +
> + if (mnt_flags & MNT_READONLY)
> + return -EROFS;
> +
> + if (!(topmost_mnt->mnt_sb->s_flags & MS_WHITEOUT))
> + return -EINVAL;
> +

Is there a need to check fallthru, umm ... that probably doesn't
apply for the ROOT(), right?

> + /* XXX top level mount should only be mounted once */
> +
> + return 0;
> +}
> +
> +void put_union_sb(struct super_block *sb)
> +{
> + struct vfsmount *mnt;
> + int i;
> +
> + if (sb->s_vfs_union_mnts) {
> + for (i = 0; i < sb->s_vfs_union_count; i++) {
> + mnt = sb->s_vfs_union_mnts[i];
> + if (mnt) {
> + dec_hard_readonly_users(mnt);
> + mntput(mnt);
> + }
> + }
> + kfree(sb->s_vfs_union_mnts);
> + }
> +}
> +
> +static void cleanup_mnt_union(struct vfsmount *topmost_mnt)
> +{
> + d_free_unions(topmost_mnt->mnt_root);
> + put_union_sb(topmost_mnt->mnt_sb);
> +}
> +
> +/**
> + * prepare_mnt_union - do setup necessary for a union mount
> + *
> + * @topmost_mnt: vfsmount of topmost layer
> + * @mntpnt: path of requested mountpoint
> + *
> + * A union mount clones the underlying read-only mounts and keeps them
> + * in its own internal list of of vfsmounts, hanging off the
> + * superblock. The first underlying mount (at @mntpnt) has passed
> + * check_mnt_union(), so we know we have at least one layer of union
> + * mount underneath this one. We union every underlying file system
> + * that is mounted on the same mountpoint (well, pathname) and
> + * read-only.

Last sentence looks a bit odd, would this be better?

We union every underlying file system that is mounted read-only on the
same mountpoint (well, pathname).

> + *
> + * XXX - Maybe should take # of layers to go down as an argument. But
> + * how to pass this in through mount options? All solutions look ugly.
> + */
> +
> +static int prepare_mnt_union(struct vfsmount *topmost_mnt, struct path *mntpnt)
> +{
> + struct vfsmount *mnt;
> + struct super_block *sb = topmost_mnt->mnt_sb;
> + struct union_dir **next_ud;
> + struct path upper, lower, this_layer;
> + int i;
> + int err;
> +
> + /* Count the mounts to be unioned. */
> + BUG_ON(sb->s_vfs_union_count != 0);
> + this_layer = *mntpnt;
> + while(check_mnt_union(&this_layer, topmost_mnt, MNT_UNION) == 0) {
> + sb->s_vfs_union_count++;
> + /* Where is this layer mounted? See if we can union that. */
> + this_layer.dentry = this_layer.mnt->mnt_mountpoint;
> + this_layer.mnt = this_layer.mnt->mnt_parent;
> + }
> + BUG_ON(sb->s_vfs_union_count == 0);
> +
> + /*
> + * Allocate an array of pointers to vfsmounts. We use this in
> + * deactivate_super() to free the underlying mounts when the
> + * topmost layer of a union mount loses its last reference.
> + *
> + * XXX - can't we link through mnt_child or mnt_hash instead?
> + * Neither is in use when a vfsmount is dangling off a union
> + * mounted superblock and therefore not part of the vfsmount
> + * tree.
> + */
> + err = -ENOMEM;
> + sb->s_vfs_union_mnts = kzalloc(sb->s_vfs_union_count *
> + sizeof (*sb->s_vfs_union_mnts),
> + GFP_KERNEL);
> + if (!sb->s_vfs_union_mnts)
> + goto out;
> +
> + /* Clone the mounts */
> + mnt = mntpnt->mnt;
> + for (i = 0; i < sb->s_vfs_union_count; i++) {
> + sb->s_vfs_union_mnts[i] = clone_mnt(mnt, mnt->mnt_root, CL_PRIVATE);
> + if (!sb->s_vfs_union_mnts[i])
> + goto out;
> + inc_hard_readonly_users(mnt);
> + /* XXX set mountpoint or otherwise manipulate cloned mnt? */
> + mnt = mnt->mnt_parent;
> + }
> +
> + /* Build the union stack for the root dir */
> + upper.mnt = topmost_mnt;
> + upper.dentry = topmost_mnt->mnt_root;
> + next_ud = &topmost_mnt->mnt_root->d_union_dir;
> + for (i = 0; i < sb->s_vfs_union_count; i++) {
> + mnt = sb->s_vfs_union_mnts[i];
> + lower.mnt = mntget(mnt);
> + lower.dentry = dget(mnt->mnt_root);
> + err = union_add_dir(&upper, &lower, next_ud);
> + if (err)
> + goto out;
> + next_ud = &lower.dentry->d_union_dir;
> + upper = lower;
> + }
> +
> + return 0;
> +out:
> + cleanup_mnt_union(topmost_mnt);
> + return err;
> +}
> +
> /*
> * @source_mnt : mount tree to be attached
> * @nd : place the mount tree @source_mnt is attached
> @@ -1411,9 +1600,16 @@ static int attach_recursive_mnt(struct vfsmount *source_mnt,
> if (err)
> goto out;
> }
> +
> + if (!parent_path && IS_MNT_UNION(source_mnt)) {
> + err = prepare_mnt_union(source_mnt, path);
> + if (err)
> + goto out_cleanup_ids;
> + }
> +
> err = propagate_mnt(dest_mnt, dest_dentry, source_mnt, &tree_list);
> if (err)
> - goto out_cleanup_ids;
> + goto out_cleanup_union;
>
> spin_lock(&vfsmount_lock);
>
> @@ -1437,6 +1633,9 @@ static int attach_recursive_mnt(struct vfsmount *source_mnt,
> spin_unlock(&vfsmount_lock);
> return 0;
>
> + out_cleanup_union:
> + if (IS_MNT_UNION(source_mnt))
> + cleanup_mnt_union(source_mnt);
> out_cleanup_ids:
> if (IS_MNT_SHARED(dest_mnt))
> cleanup_group_ids(source_mnt, NULL);
> @@ -1490,6 +1689,17 @@ static int do_change_type(struct path *path, int flag)
> return -EINVAL;
>
> down_write(&namespace_sem);
> +
> + /*
> + * Mounts of file systems with read-only users can't deal with
> + * mount/umount propagation events - it's the moral equivalent
> + * of rm -rf dir/ or the like.
> + */
> + if (sb_is_hard_readonly(mnt->mnt_sb)) {
> + err = -EROFS;
> + goto out_unlock;
> + }
> +
> if (type == MS_SHARED) {
> err = invent_group_ids(mnt, recurse);
> if (err)
> @@ -1527,6 +1737,9 @@ static int do_loopback(struct path *path, char *old_name,
> err = -EINVAL;
> if (IS_MNT_UNBINDABLE(old_path.mnt))
> goto out;
> + /* Mount part of a union mount elsewhere? The mind boggles. */
> + if (IS_MNT_UNION(old_path.mnt))
> + goto out;
>
> if (!check_mnt(path->mnt) || !check_mnt(old_path.mnt))
> goto out;
> @@ -1548,7 +1761,6 @@ static int do_loopback(struct path *path, char *old_name,
> spin_unlock(&vfsmount_lock);
> release_mounts(&umount_list);
> }
> -
> out:
> up_write(&namespace_sem);
> path_put(&old_path);
> @@ -1589,6 +1801,17 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
> if (!check_mnt(path->mnt))
> return -EINVAL;
>
> + if (mnt_flags & MNT_UNION)
> + return -EINVAL;
> +
> + if ((path->mnt->mnt_flags & MNT_UNION) &&
> + !(mnt_flags & MNT_UNION))
> + return -EINVAL;
> +
> + if ((path->mnt->mnt_flags & MNT_UNION) &&
> + (mnt_flags & MNT_READONLY))
> + return -EINVAL;
> +
> if (path->dentry != path->mnt->mnt_root)
> return -EINVAL;
>
> @@ -1753,6 +1976,10 @@ int do_add_mount(struct vfsmount *newmnt, struct path *path,
> if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
> goto unlock;
>
> + err = check_mnt_union(path, newmnt, mnt_flags);
> + if (err)
> + goto unlock;
> +
> newmnt->mnt_flags = mnt_flags;
> if ((err = graft_tree(newmnt, path)))
> goto unlock;
> diff --git a/fs/super.c b/fs/super.c
> index 6add39b..2ade113 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -197,6 +197,7 @@ void deactivate_super(struct super_block *s)
> down_write(&s->s_umount);
> fs->kill_sb(s);
> put_filesystem(fs);
> + put_union_sb(s);
> put_super(s);
> }
> }
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 32e6988..cc2934d 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1396,6 +1396,9 @@ struct super_block {
> */
> int s_hard_readonly_users;
>
> + /* Array of vfsmounts that are part of this union mount */
> + struct vfsmount **s_vfs_union_mnts;
> + int s_vfs_union_count;
> };
>
> extern struct timespec current_fs_time(struct super_block *sb);
> diff --git a/include/linux/mount.h b/include/linux/mount.h
> index 0302703..17d3d27 100644
> --- a/include/linux/mount.h
> +++ b/include/linux/mount.h
> @@ -136,4 +136,6 @@ extern void mark_mounts_for_expiry(struct list_head *mounts);
>
> extern dev_t name_to_dev_t(char *name);
>
> +extern void put_union_sb(struct super_block *sb);
> +
> #endif /* _LINUX_MOUNT_H */
> --
> 1.6.3.3
>
> --
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