xfs: count free space btree blocks when scrubbing pre-lazysbcount fses

Since agf_btreeblks didn't exist before the lazysbcount feature, the fs
summary count scrubber needs to walk the free space btrees to determine
the amount of space being used by those btrees.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Gao Xiang <hsiangkao@redhat.com>

fs/xfs/scrub/fscounters.c

@@ -13,6 +13,7 @@
 #include "xfs_alloc.h"
 #include "xfs_ialloc.h"
 #include "xfs_health.h"
+#include "xfs_btree.h"
 #include "scrub/scrub.h"
 #include "scrub/common.h"
 #include "scrub/trace.h"
@@ -143,6 +144,35 @@ xchk_setup_fscounters(
 	return xchk_trans_alloc(sc, 0);
 }
 
+/* Count free space btree blocks manually for pre-lazysbcount filesystems. */
+static int
+xchk_fscount_btreeblks(
+	struct xfs_scrub	*sc,
+	struct xchk_fscounters	*fsc,
+	xfs_agnumber_t		agno)
+{
+	xfs_extlen_t		blocks;
+	int			error;
+
+	error = xchk_ag_init(sc, agno, &sc->sa);
+	if (error)
+		return error;
+
+	error = xfs_btree_count_blocks(sc->sa.bno_cur, &blocks);
+	if (error)
+		goto out_free;
+	fsc->fdblocks += blocks - 1;
+
+	error = xfs_btree_count_blocks(sc->sa.cnt_cur, &blocks);
+	if (error)
+		goto out_free;
+	fsc->fdblocks += blocks - 1;
+
+out_free:
+	xchk_ag_free(sc, &sc->sa);
+	return error;
+}
+
 /*
  * Calculate what the global in-core counters ought to be from the incore
  * per-AG structure.  Callers can compare this to the actual in-core counters
@@ -182,8 +212,15 @@ xchk_fscount_aggregate_agcounts(
 		/* Add up the free/freelist/bnobt/cntbt blocks */
 		fsc->fdblocks += pag->pagf_freeblks;
 		fsc->fdblocks += pag->pagf_flcount;
-		if (xfs_sb_version_haslazysbcount(&sc->mp->m_sb))
+		if (xfs_sb_version_haslazysbcount(&sc->mp->m_sb)) {
 			fsc->fdblocks += pag->pagf_btreeblks;
+		} else {
+			error = xchk_fscount_btreeblks(sc, fsc, agno);
+			if (error) {
+				xfs_perag_put(pag);
+				break;
+			}
+		}
 
 		/*
 		 * Per-AG reservations are taken out of the incore counters,