[PATCH] Btrfs: fix broken nocow after balance

[Liu Bo <bo.li.liu@oracle.com>]

Balance will create reloc_root for each fs root, and it's going to
record last_snapshot to filter shared blocks.  The side effect of
setting last_snapshot is to break nocow attributes of files.

So it turns out that checking last_snapshot does not always ensure that
a node/leaf/file_extent is shared.

That's why shared node/leaf needs to search extent tree for number of references
even after having checked last_snapshot, and updating fs/file tree works
top-down so the children will always know how many references parents put
on them at the moment of checking shared status.

However, our nocow path does something different, it'll firstly check
if the file extent is shared, then update fs/file tree by updating inode.
This ends up that the related extent record to the file extent may don't
have actual multiple references when checking shared status.

------------------------------------------------
fs_root snap
   \    /
    leaf ==> refs=2
     |
  file_extent ==> refs=1(but actually refs is 2)

After updating fs tree(or snapshot if snapshot is not RO), it'll be

fs root snap
  \      /
  cow  leaf
    \  /
  file_extent ==> refs=2(we do have two parents)
------------------------------------------------

So it'll be confused by last_snapshot from balance to think that the file
extent is now shared.

There are actually a couple of ways to address it, but updating fs/file tree
firstly might be the easiest and cleanest one.  With this, updating fs/file
tree will at least make a delayed ref if the file extent is really shared by
several parents, we can make nocow happy again without having to check confusing
last_snapshot.

Reported-by: Kyle Gates <kylegates@hotmail.com>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
---
 fs/btrfs/extent-tree.c |    4 ----
 fs/btrfs/inode.c       |    2 +-
 2 files changed, 1 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index df472ab..d24c26c 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -2856,10 +2856,6 @@ static noinline int check_committed_ref(struct btrfs_trans_handle *trans,
 	    btrfs_extent_inline_ref_size(BTRFS_EXTENT_DATA_REF_KEY))
 		goto out;
 
-	if (btrfs_extent_generation(leaf, ei) <=
-	    btrfs_root_last_snapshot(&root->root_item))
-		goto out;
-
 	iref = (struct btrfs_extent_inline_ref *)(ei + 1);
 	if (btrfs_extent_inline_ref_type(leaf, iref) !=
 	    BTRFS_EXTENT_DATA_REF_KEY)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 23c596c..0dc5c7d 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1253,7 +1253,7 @@ static noinline int run_delalloc_nocow(struct inode *inode,
 	cur_offset = start;
 	while (1) {
 		ret = btrfs_lookup_file_extent(trans, root, path, ino,
-					       cur_offset, 0);
+					       cur_offset, 1);
 		if (ret < 0) {
 			btrfs_abort_transaction(trans, root, ret);
 			goto error;
-- 
1.7.7