[PATCH v2] Btrfs: kill BUG_ON in do_relocation

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

While updating btree, we try to push items between sibling
nodes/leaves in order to keep height as low as possible.
But we don't memset the original places with zero when
pushing items so that we could end up leaving stale content
in nodes/leaves.  One may read the above stale content by
increasing btree blocks' @nritems.

One case I've come across is that in fs tree, a leaf has two
parent nodes, hence running balance ends up with processing
this leaf with two parent nodes, but it can only reach the
valid parent node through btrfs_search_slot, so it'd be like,

do_relocation
    for P in all parent nodes of block A:
        if !P->eb:
            btrfs_search_slot(key);   --> get path from P to A.
        if lowest:
            BUG_ON(A->bytenr != bytenr of A recorded in P);
        btrfs_cow_block(P, A);   --> change A's bytenr in P.

After btrfs_cow_block, P has the new bytenr of A, but with the
same @key, we get the same path again, and get panic by BUG_ON.

Note that this is only happening in a corrupted fs, for a
regular fs in which we have correct @nritems so that we won't
read stale content in any case.

Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
---
v2: - use new internal error EFSCORRUPTED as "Filesystem is corrupted",
      suggested by David Sterba.

 fs/btrfs/ctree.h      | 2 ++
 fs/btrfs/relocation.c | 9 ++++++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 67d71c0..8b23410 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -127,6 +127,8 @@ static inline unsigned long btrfs_chunk_item_size(int num_stripes)
 #define BTRFS_OLD_BACKREF_REV		0
 #define BTRFS_MIXED_BACKREF_REV		1
 
+#define EFSCORRUPTED			EUCLEAN /* Filesystem is corrupted */
+
 /*
  * every tree block (leaf or node) starts with this header.
  */
diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index c0c13dc..6f8b952 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -2712,7 +2712,14 @@ static int do_relocation(struct btrfs_trans_handle *trans,
 
 		bytenr = btrfs_node_blockptr(upper->eb, slot);
 		if (lowest) {
-			BUG_ON(bytenr != node->bytenr);
+			if (bytenr != node->bytenr) {
+				btrfs_err(root->fs_info,
+		"lowest leaf/node mismatch: bytenr %llu node->bytenr %llu slot %d upper %llu",
+					  bytenr, node->bytenr, slot,
+					  upper->eb->start);
+				err = -EFSCORRUPTED;
+				goto next;
+			}
 		} else {
 			if (node->eb->start == bytenr)
 				goto next;
-- 
2.5.5