Btrfs: change how subvolumes are organized

btrfs allows subvolumes and snapshots anywhere in the directory tree.
If we snapshot a subvolume that contains a link to other subvolume
called subvolA, subvolA can be accessed through both the original
subvolume and the snapshot. This is similar to creating hard link to
directory, and has the very similar problems.

The aim of this patch is enforcing there is only one access point to
each subvolume. Only the first directory entry (the one added when
the subvolume/snapshot was created) is treated as valid access point.
The first directory entry is distinguished by checking root forward
reference. If the corresponding root forward reference is missing,
we know the entry is not the first one.

This patch also adds snapshot/subvolume rename support, the code
allows rename subvolume link across subvolumes.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/fs/btrfs/orphan.c b/fs/btrfs/orphan.c
index 3c0d52af4f80..79cba5fbc28e 100644
--- a/fs/btrfs/orphan.c
+++ b/fs/btrfs/orphan.c
@@ -65,3 +65,23 @@ out:
         btrfs_free_path(path);
         return ret;
 }
+
+int btrfs_find_orphan_item(struct btrfs_root *root, u64 offset)
+{
+        struct btrfs_path *path;
+        struct btrfs_key key;
+        int ret;
+
+        key.objectid = BTRFS_ORPHAN_OBJECTID;
+        key.type = BTRFS_ORPHAN_ITEM_KEY;
+        key.offset = offset;
+
+        path = btrfs_alloc_path();
+        if (!path)
+                return -ENOMEM;
+
+        ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+
+        btrfs_free_path(path);
+        return ret;
+}