[PATCH V2] xfs: scrub: Disable check for unoptimized data fork bmbt node

[Chandan Babu R <chandanrlinux@gmail.com>]

xchk_btree_check_minrecs() checks if the contents of the immediate child of a
bmbt root block can fit within the root block. This check could fail on inodes
with an attr fork since xfs_bmap_add_attrfork_btree() used to demote the
current root node of the data fork as the child of a newly allocated root node
if it found that the size of "struct xfs_btree_block" along with the space
required for records exceeded that of space available in the data fork.

xfs_bmap_add_attrfork_btree() should have used "struct xfs_bmdr_block" instead
of "struct xfs_btree_block" for the above mentioned space requirement
calculation. This commit disables the check for unoptimized (in terms of
disk space usage) data fork bmbt trees since there could be filesystems
in use that already have such a layout.

Suggested-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
---
Changelog:
 V1 -> V2:
   1. Apply "minimum records check" restriction only to BMBT btrees.
   
 fs/xfs/scrub/btree.c | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/scrub/btree.c b/fs/xfs/scrub/btree.c
index debf392e0515..a94bd8122c60 100644
--- a/fs/xfs/scrub/btree.c
+++ b/fs/xfs/scrub/btree.c
@@ -9,6 +9,7 @@
 #include "xfs_format.h"
 #include "xfs_trans_resv.h"
 #include "xfs_mount.h"
+#include "xfs_inode.h"
 #include "xfs_btree.h"
 #include "scrub/scrub.h"
 #include "scrub/common.h"
@@ -442,6 +443,30 @@ xchk_btree_check_owner(
 	return xchk_btree_check_block_owner(bs, level, XFS_BUF_ADDR(bp));
 }
 
+/* Decide if we want to check minrecs of a btree block in the inode root. */
+static inline bool
+xchk_btree_check_iroot_minrecs(
+	struct xchk_btree	*bs)
+{
+	/*
+	 * xfs_bmap_add_attrfork_btree had an implementation bug wherein it
+	 * would miscalculate the space required for the data fork bmbt root
+	 * when adding an attr fork, and promote the iroot contents to an
+	 * external block unnecessarily.  This went unnoticed for many years
+	 * until scrub found filesystems in this state.  Inode rooted btrees are
+	 * not supposed to have immediate child blocks that are small enough
+	 * that the contents could fit in the inode root, but we can't fail
+	 * existing filesystems, so instead we disable the check for data fork
+	 * bmap btrees when there's an attr fork.
+	 */
+	if (bs->cur->bc_btnum == XFS_BTNUM_BMAP &&
+	    bs->cur->bc_ino.whichfork == XFS_DATA_FORK &&
+	    XFS_IFORK_Q(bs->sc->ip))
+		return false;
+
+	return true;
+}
+
 /*
  * Check that this btree block has at least minrecs records or is one of the
  * special blocks that don't require that.
@@ -475,8 +500,9 @@ xchk_btree_check_minrecs(
 
 		root_block = xfs_btree_get_block(cur, root_level, &root_bp);
 		root_maxrecs = cur->bc_ops->get_dmaxrecs(cur, root_level);
-		if (be16_to_cpu(root_block->bb_numrecs) != 1 ||
-		    numrecs <= root_maxrecs)
+		if (xchk_btree_check_iroot_minrecs(bs) &&
+		    (be16_to_cpu(root_block->bb_numrecs) != 1 ||
+		     numrecs <= root_maxrecs))
 			xchk_btree_set_corrupt(bs->sc, cur, level);
 		return;
 	}
-- 
2.29.2