xfs: use deferred frees to reap old btree blocks

Use deferred frees (EFIs) to reap the blocks of a btree that we just
replaced.  This helps us to shrink the window in which those old blocks
could be lost due to a system crash, though we try to flush the EFIs
every few hundred blocks so that we don't also overflow the transaction
reservations during and after we commit the new btree.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/scrub/reap.c

@@ -26,6 +26,7 @@
 #include "xfs_ag_resv.h"
 #include "xfs_quota.h"
 #include "xfs_qm.h"
+#include "xfs_bmap.h"
 #include "scrub/scrub.h"
 #include "scrub/common.h"
 #include "scrub/trace.h"
@@ -81,6 +82,9 @@ struct xrep_reap_state {
 	/* Reverse mapping owner and metadata reservation type. */
 	const struct xfs_owner_info	*oinfo;
 	enum xfs_ag_resv_type		resv;
+
+	/* Number of deferred reaps attached to the current transaction. */
+	unsigned int			deferred;
 };
 
 /* Put a block back on the AGFL. */
@@ -165,6 +169,7 @@ xrep_reap_block(
 	xfs_agnumber_t			agno;
 	xfs_agblock_t			agbno;
 	bool				has_other_rmap;
+	bool				need_roll = true;
 	int				error;
 
 	agno = XFS_FSB_TO_AGNO(sc->mp, fsbno);
@@ -207,13 +212,25 @@ xrep_reap_block(
 		xrep_block_reap_binval(sc, fsbno);
 		error = xrep_put_freelist(sc, agbno);
 	} else {
+		/*
+		 * Use deferred frees to get rid of the old btree blocks to try
+		 * to minimize the window in which we could crash and lose the
+		 * old blocks.  However, we still need to roll the transaction
+		 * every 100 or so EFIs so that we don't exceed the log
+		 * reservation.
+		 */
 		xrep_block_reap_binval(sc, fsbno);
-		error = xfs_free_extent(sc->tp, sc->sa.pag, agbno, 1, rs->oinfo,
-				rs->resv);
+		error = __xfs_free_extent_later(sc->tp, fsbno, 1, rs->oinfo,
+				rs->resv, true);
+		if (error)
+			return error;
+		rs->deferred++;
+		need_roll = rs->deferred > 100;
 	}
-	if (error)
+	if (error || !need_roll)
 		return error;
 
+	rs->deferred = 0;
 	return xrep_roll_ag_trans(sc);
 }
 
@@ -230,8 +247,13 @@ xrep_reap_extents(
 		.oinfo			= oinfo,
 		.resv			= type,
 	};
+	int				error;
 
 	ASSERT(xfs_has_rmapbt(sc->mp));
 
-	return xbitmap_walk_bits(bitmap, xrep_reap_block, &rs);
+	error = xbitmap_walk_bits(bitmap, xrep_reap_block, &rs);
+	if (error || rs.deferred == 0)
+		return error;
+
+	return xrep_roll_ag_trans(sc);
 }