xfs: accumulate iextent records when checking bmap

Currently, the bmap scrubber checks file fork mappings individually.  In
the case that the file uses multiple mappings to a single contiguous
piece of space, the scrubber repeatedly locks the AG to check the
existence of a reverse mapping that overlaps this file mapping.  If the
reverse mapping starts before or ends after the mapping we're checking,
it will also crawl around in the bmbt checking correspondence for
adjacent extents.

This is not very time efficient because it does the crawling while
holding the AGF buffer, and checks the middle mappings multiple times.
Instead, create a custom iextent record iterator function that combines
multiple adjacent allocated mappings into one large incore bmbt record.
This is feasible because the incore bmbt record length is 64-bits wide.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

Author: Darrick J. Wong

fs/xfs/libxfs/xfs_bmap.h

@@ -145,7 +145,7 @@ static inline int xfs_bmapi_whichfork(uint32_t bmapi_flags)
 	{ BMAP_COWFORK,		"COW" }
 
 /* Return true if the extent is an allocated extent, written or not. */
-static inline bool xfs_bmap_is_real_extent(struct xfs_bmbt_irec *irec)
+static inline bool xfs_bmap_is_real_extent(const struct xfs_bmbt_irec *irec)
 {
 	return irec->br_startblock != HOLESTARTBLOCK &&
 		irec->br_startblock != DELAYSTARTBLOCK &&

fs/xfs/scrub/bmap.c

@@ -165,48 +165,6 @@ xchk_bmap_get_rmap(
 	return has_rmap;
 }
 
-static inline bool
-xchk_bmap_has_prev(
-	struct xchk_bmap_info	*info,
-	struct xfs_bmbt_irec	*irec)
-{
-	struct xfs_bmbt_irec	got;
-	struct xfs_ifork	*ifp;
-
-	ifp = xfs_ifork_ptr(info->sc->ip, info->whichfork);
-
-	if (!xfs_iext_peek_prev_extent(ifp, &info->icur, &got))
-		return false;
-	if (got.br_startoff + got.br_blockcount != irec->br_startoff)
-		return false;
-	if (got.br_startblock + got.br_blockcount != irec->br_startblock)
-		return false;
-	if (got.br_state != irec->br_state)
-		return false;
-	return true;
-}
-
-static inline bool
-xchk_bmap_has_next(
-	struct xchk_bmap_info	*info,
-	struct xfs_bmbt_irec	*irec)
-{
-	struct xfs_bmbt_irec	got;
-	struct xfs_ifork	*ifp;
-
-	ifp = xfs_ifork_ptr(info->sc->ip, info->whichfork);
-
-	if (!xfs_iext_peek_next_extent(ifp, &info->icur, &got))
-		return false;
-	if (irec->br_startoff + irec->br_blockcount != got.br_startoff)
-		return false;
-	if (irec->br_startblock + irec->br_blockcount != got.br_startblock)
-		return false;
-	if (got.br_state != irec->br_state)
-		return false;
-	return true;
-}
-
 /* Make sure that we have rmapbt records for this extent. */
 STATIC void
 xchk_bmap_xref_rmap(
@@ -277,31 +235,20 @@ xchk_bmap_xref_rmap(
 				irec->br_startoff);
 
 	/*
-	 * If the rmap starts before this bmbt record, make sure there's a bmbt
-	 * record for the previous offset that is contiguous with this mapping.
-	 * Skip this for CoW fork extents because the refcount btree (and not
-	 * the inode) is the ondisk owner for those extents.
-	 */
-	if (info->whichfork != XFS_COW_FORK && rmap.rm_startblock < agbno &&
-	    !xchk_bmap_has_prev(info, irec)) {
-		xchk_fblock_xref_set_corrupt(info->sc, info->whichfork,
-				irec->br_startoff);
-		return;
-	}
-
-	/*
-	 * If the rmap ends after this bmbt record, make sure there's a bmbt
-	 * record for the next offset that is contiguous with this mapping.
-	 * Skip this for CoW fork extents because the refcount btree (and not
-	 * the inode) is the ondisk owner for those extents.
+	 * The rmap must correspond exactly with this bmbt record.  Skip this
+	 * for CoW fork extents because the refcount btree (and not the inode)
+	 * is the ondisk owner for those extents.
 	 */
-	rmap_end = (unsigned long long)rmap.rm_startblock + rmap.rm_blockcount;
-	if (info->whichfork != XFS_COW_FORK &&
-	    rmap_end > agbno + irec->br_blockcount &&
-	    !xchk_bmap_has_next(info, irec)) {
-		xchk_fblock_xref_set_corrupt(info->sc, info->whichfork,
-				irec->br_startoff);
-		return;
+	if (info->whichfork != XFS_COW_FORK) {
+		if (rmap.rm_startblock != agbno)
+			xchk_fblock_xref_set_corrupt(info->sc, info->whichfork,
+					irec->br_startoff);
+
+		rmap_end = (unsigned long long)rmap.rm_startblock +
+					       rmap.rm_blockcount;
+		if (rmap_end != agbno + irec->br_blockcount)
+			xchk_fblock_xref_set_corrupt(info->sc, info->whichfork,
+					irec->br_startoff);
 	}
 }
 
@@ -428,15 +375,7 @@ xchk_bmap_iextent(
 
 	xchk_bmap_dirattr_extent(ip, info, irec);
 
-	/* There should never be a "hole" extent in either extent list. */
-	if (irec->br_startblock == HOLESTARTBLOCK)
-		xchk_fblock_set_corrupt(info->sc, info->whichfork,
-				irec->br_startoff);
-
 	/* Make sure the extent points to a valid place. */
-	if (irec->br_blockcount > XFS_MAX_BMBT_EXTLEN)
-		xchk_fblock_set_corrupt(info->sc, info->whichfork,
-				irec->br_startoff);
 	if (info->is_rt &&
 	    !xfs_verify_rtext(mp, irec->br_startblock, irec->br_blockcount))
 		xchk_fblock_set_corrupt(info->sc, info->whichfork,
@@ -740,6 +679,90 @@ xchk_bmap_iextent_delalloc(
 				irec->br_startoff);
 }
 
+/* Decide if this individual fork mapping is ok. */
+static bool
+xchk_bmap_iext_mapping(
+	struct xchk_bmap_info		*info,
+	const struct xfs_bmbt_irec	*irec)
+{
+	/* There should never be a "hole" extent in either extent list. */
+	if (irec->br_startblock == HOLESTARTBLOCK)
+		return false;
+	if (irec->br_blockcount > XFS_MAX_BMBT_EXTLEN)
+		return false;
+	return true;
+}
+
+/* Are these two mappings contiguous with each other? */
+static inline bool
+xchk_are_bmaps_contiguous(
+	const struct xfs_bmbt_irec	*b1,
+	const struct xfs_bmbt_irec	*b2)
+{
+	/* Don't try to combine unallocated mappings. */
+	if (!xfs_bmap_is_real_extent(b1))
+		return false;
+	if (!xfs_bmap_is_real_extent(b2))
+		return false;
+
+	/* Does b2 come right after b1 in the logical and physical range? */
+	if (b1->br_startoff + b1->br_blockcount != b2->br_startoff)
+		return false;
+	if (b1->br_startblock + b1->br_blockcount != b2->br_startblock)
+		return false;
+	if (b1->br_state != b2->br_state)
+		return false;
+	return true;
+}
+
+/*
+ * Walk the incore extent records, accumulating consecutive contiguous records
+ * into a single incore mapping.  Returns true if @irec has been set to a
+ * mapping or false if there are no more mappings.  Caller must ensure that
+ * @info.icur is zeroed before the first call.
+ */
+static int
+xchk_bmap_iext_iter(
+	struct xchk_bmap_info	*info,
+	struct xfs_bmbt_irec	*irec)
+{
+	struct xfs_bmbt_irec	got;
+	struct xfs_ifork	*ifp;
+
+	ifp = xfs_ifork_ptr(info->sc->ip, info->whichfork);
+
+	/* Advance to the next iextent record and check the mapping. */
+	xfs_iext_next(ifp, &info->icur);
+	if (!xfs_iext_get_extent(ifp, &info->icur, irec))
+		return false;
+
+	if (!xchk_bmap_iext_mapping(info, irec)) {
+		xchk_fblock_set_corrupt(info->sc, info->whichfork,
+				irec->br_startoff);
+		return false;
+	}
+
+	/*
+	 * Iterate subsequent iextent records and merge them with the one
+	 * that we just read, if possible.
+	 */
+	while (xfs_iext_peek_next_extent(ifp, &info->icur, &got)) {
+		if (!xchk_are_bmaps_contiguous(irec, &got))
+			break;
+
+		if (!xchk_bmap_iext_mapping(info, &got)) {
+			xchk_fblock_set_corrupt(info->sc, info->whichfork,
+					got.br_startoff);
+			return false;
+		}
+
+		irec->br_blockcount += got.br_blockcount;
+		xfs_iext_next(ifp, &info->icur);
+	}
+
+	return true;
+}
+
 /*
  * Scrub an inode fork's block mappings.
  *
@@ -819,9 +842,15 @@ xchk_bmap(
 	if (!xchk_fblock_process_error(sc, whichfork, 0, &error))
 		goto out;
 
-	/* Scrub extent records. */
-	ifp = xfs_ifork_ptr(ip, whichfork);
-	for_each_xfs_iext(ifp, &info.icur, &irec) {
+	/*
+	 * Scrub extent records.  We use a special iterator function here that
+	 * combines adjacent mappings if they are logically and physically
+	 * contiguous.   For large allocations that require multiple bmbt
+	 * records, this reduces the number of cross-referencing calls, which
+	 * reduces runtime.  Cross referencing with the rmap is simpler because
+	 * the rmap must match the combined mapping exactly.
+	 */
+	while (xchk_bmap_iext_iter(&info, &irec)) {
 		if (xchk_should_terminate(sc, &error) ||
 		    (sc->sm->sm_flags & XFS_SCRUB_OFLAG_CORRUPT))
 			goto out;