[PATCH] btrfs: send: fix a failure when looking for data backrefs after relocation

[fdmanana@kernel.org]

From: Filipe Manana <fdmanana@suse.com>

During a send, when trying to find roots from which to clone data extents,
if the leaf of our file extent item was obtained before relocation for a
data block group finished, we can end up trying to lookup for backrefs
for an extent location (file extent item's disk_bytenr) that is not in
use anymore. That is, the extent was reallocated and the transaction used
for the relocation was committed. This makes the backref lookup not find
anything and we fail at find_extent_clone() with -EIO and log an error
message like the following:

  [ 7642.897365] BTRFS error (device sdc): did not find backref in send_root. inode=881, offset=2592768, disk_byte=1292025856 found extent=1292025856

This is because we are checking if relocation happened after we check if
we found the backref for the file extent item we are processing. We should
do it before, and in case relocation happened, do not attempt to clone and
instead fallback to issuing write commands, which will read the correct
data from the new extent location. The current check is being done too
late, so fix this by moving it to right after we do the backref lookup and
before checking if we found our own backref.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---

David, this can be squashed into the patch:

   "btrfs: make send work with concurrent block group relocation"

 fs/btrfs/send.c | 42 +++++++++++++++++++++---------------------
 1 file changed, 21 insertions(+), 21 deletions(-)

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index f0015b5cf4b1..3fc144b8c0d8 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -1431,6 +1431,26 @@ static int find_extent_clone(struct send_ctx *sctx,
 	if (ret < 0)
 		goto out;
 
+	down_read(&fs_info->commit_root_sem);
+	if (fs_info->last_reloc_trans > sctx->last_reloc_trans) {
+		/*
+		 * A transaction commit for a transaction in which block group
+		 * relocation was done just happened.
+		 * The disk_bytenr of the file extent item we processed is
+		 * possibly stale, referring to the extent's location before
+		 * relocation. So act as if we haven't found any clone sources
+		 * and fallback to write commands, which will read the correct
+		 * data from the new extent location. Otherwise we will fail
+		 * below because we haven't found our own back reference or we
+		 * could be getting incorrect sources in case the old extent
+		 * was already reallocated after the relocation.
+		 */
+		up_read(&fs_info->commit_root_sem);
+		ret = -ENOENT;
+		goto out;
+	}
+	up_read(&fs_info->commit_root_sem);
+
 	if (!backref_ctx.found_itself) {
 		/* found a bug in backref code? */
 		ret = -EIO;
@@ -1444,28 +1464,8 @@ static int find_extent_clone(struct send_ctx *sctx,
 		    "find_extent_clone: data_offset=%llu, ino=%llu, num_bytes=%llu, logical=%llu",
 		    data_offset, ino, num_bytes, logical);
 
-	if (backref_ctx.found > 0) {
-		down_read(&fs_info->commit_root_sem);
-		if (fs_info->last_reloc_trans > sctx->last_reloc_trans) {
-			/*
-			 * A transaction commit for a transaction in which block
-			 * group relocation was done just happened.
-			 * The disk_bytenr of the file extent item we processed
-			 * is possibly stale, referring to the extent's location
-			 * before relocation, so act as if we haven't found any
-			 * clone sources - otherwise we could end up later issuing
-			 * clone operations that could leave the receiver with
-			 * incorrect data, in case the old disk_bytenr got
-			 * reallocated for another extent.
-			 */
-			up_read(&fs_info->commit_root_sem);
-			ret = -ENOENT;
-			goto out;
-		}
-		up_read(&fs_info->commit_root_sem);
-	} else {
+	if (!backref_ctx.found)
 		btrfs_debug(fs_info, "no clones found");
-	}
 
 	cur_clone_root = NULL;
 	for (i = 0; i < sctx->clone_roots_cnt; i++) {
-- 
2.33.0