dm: make flush bios explicitly sync

Commit b685d3d65ac7 ("block: treat REQ_FUA and REQ_PREFLUSH as
synchronous") removed REQ_SYNC flag from WRITE_{FUA|PREFLUSH|...}
definitions. generic_make_request_checks() however strips REQ_FUA and
REQ_PREFLUSH flags from a bio when the storage doesn't report volatile
write cache and thus write effectively becomes asynchronous which can
lead to performance regressions.

Fix the problem by making sure all bios which are synchronous are
properly marked with REQ_SYNC.

Fixes: b685d3d65ac7 ("block: treat REQ_FUA and REQ_PREFLUSH as synchronous")
Cc: [e-mail]
Signed-off-by: Jan Kara <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-bufio.c

···
 {
 	struct dm_io_request io_req = {
 		.bi_op = REQ_OP_WRITE,
-		.bi_op_flags = REQ_PREFLUSH,
+		.bi_op_flags = REQ_PREFLUSH | REQ_SYNC,
 		.mem.type = DM_IO_KMEM,
 		.mem.ptr.addr = NULL,
 		.client = c->dm_io,

drivers/md/dm-integrity.c

···
 			for (i = 0; i < commit_sections; i++)
 				rw_section_mac(ic, commit_start + i, true);
 		}
-		rw_journal(ic, REQ_OP_WRITE, REQ_FUA, commit_start, commit_sections, &io_comp);
+		rw_journal(ic, REQ_OP_WRITE, REQ_FUA | REQ_SYNC, commit_start,
+			   commit_sections, &io_comp);
 	} else {
 		unsigned to_end;
 		io_comp.in_flight = (atomic_t)ATOMIC_INIT(2);

drivers/md/dm-raid1.c

···
 	struct mirror *m;
 	struct dm_io_request io_req = {
 		.bi_op = REQ_OP_WRITE,
-		.bi_op_flags = REQ_PREFLUSH,
+		.bi_op_flags = REQ_PREFLUSH | REQ_SYNC,
 		.mem.type = DM_IO_KMEM,
 		.mem.ptr.addr = NULL,
 		.client = ms->io_client,

drivers/md/dm-snap-persistent.c

···
 	/*
 	 * Commit exceptions to disk.
 	 */
-	if (ps->valid && area_io(ps, REQ_OP_WRITE, REQ_PREFLUSH | REQ_FUA))
+	if (ps->valid && area_io(ps, REQ_OP_WRITE,
+				 REQ_PREFLUSH | REQ_FUA | REQ_SYNC))
 		ps->valid = 0;
 
 	/*

drivers/md/dm.c

···
 
 	bio_init(&md->flush_bio, NULL, 0);
 	md->flush_bio.bi_bdev = md->bdev;
-	md->flush_bio.bi_opf = REQ_OP_WRITE | REQ_PREFLUSH;
+	md->flush_bio.bi_opf = REQ_OP_WRITE | REQ_PREFLUSH | REQ_SYNC;
 
 	dm_stats_init(&md->stats);