Btrfs: do not cache rbio pages if using raid6 recover

Since raid6 recover tries all possible combinations of failed stripes,

- when raid6 rebuild algorithm is used, i.e. raid6_datap_recov() and
  raid6_2data_recov(), it may change the in-memory content of failed
  stripes, if such a raid bio is cached, a later raid write rmw or recover
  can steal @stripe_pages from it instead of reading from disks, such that
  it carries the wrong content to do write rmw or recovery and ends up
  with corruption or recovery failures.

- when raid5 rebuild algorithm is used, i.e. xor, raid bio can be cached
  because the only failed stripe which contains @rbio->bio_pages gets
  modified, others remain the same so that their in-memory content is
  consistent with their on-disk content.

This adds a check to skip caching rbio if using raid6 recover.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/raid56.c

@@ -1974,7 +1974,22 @@ cleanup:
 
 cleanup_io:
 	if (rbio->operation == BTRFS_RBIO_READ_REBUILD) {
-		if (err == BLK_STS_OK)
+		/*
+		 * - In case of two failures, where rbio->failb != -1:
+		 *
+		 *   Do not cache this rbio since the above read reconstruction
+		 *   (raid6_datap_recov() or raid6_2data_recov()) may have
+		 *   changed some content of stripes which are not identical to
+		 *   on-disk content any more, otherwise, a later write/recover
+		 *   may steal stripe_pages from this rbio and end up with
+		 *   corruptions or rebuild failures.
+		 *
+		 * - In case of single failure, where rbio->failb == -1:
+		 *
+		 *   Cache this rbio iff the above read reconstruction is
+		 *   excuted without problems.
+		 */
+		if (err == BLK_STS_OK && rbio->failb < 0)
 			cache_rbio_pages(rbio);
 		else
 			clear_bit(RBIO_CACHE_READY_BIT, &rbio->flags);