xfs: flag empty xattr leaf blocks for optimization

Empty xattr leaf blocks at offset zero are a waste of space but
otherwise harmless.  If we encounter one, flag it as an opportunity for
optimization.

If we encounter empty attr leaf blocks anywhere else in the attr fork,
that's corruption.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/scrub/attr.c

@@ -420,6 +420,17 @@ xchk_xattr_block(
 	xfs_attr3_leaf_hdr_from_disk(mp->m_attr_geo, &leafhdr, leaf);
 	hdrsize = xfs_attr3_leaf_hdr_size(leaf);
 
+	/*
+	 * Empty xattr leaf blocks mapped at block 0 are probably a byproduct
+	 * of a race between setxattr and a log shutdown.  Anywhere else in the
+	 * attr fork is a corruption.
+	 */
+	if (leafhdr.count == 0) {
+		if (blk->blkno == 0)
+			xchk_da_set_preen(ds, level);
+		else
+			xchk_da_set_corrupt(ds, level);
+	}
 	if (leafhdr.usedbytes > mp->m_attr_geo->blksize)
 		xchk_da_set_corrupt(ds, level);
 	if (leafhdr.firstused > mp->m_attr_geo->blksize)

fs/xfs/scrub/dabtree.h

@@ -37,6 +37,8 @@ bool xchk_da_process_error(struct xchk_da_btree *ds, int level, int *error);
 void xchk_da_set_corrupt(struct xchk_da_btree *ds, int level);
 void xchk_da_set_preen(struct xchk_da_btree *ds, int level);
 
+void xchk_da_set_preen(struct xchk_da_btree *ds, int level);
+
 int xchk_da_btree_hash(struct xchk_da_btree *ds, int level, __be32 *hashp);
 int xchk_da_btree(struct xfs_scrub *sc, int whichfork,
 		xchk_da_btree_rec_fn scrub_fn, void *private);