dm writecache: flush origin device when writing and cache is full

Commit d53f1fa ("dm writecache: do
direct write if the cache is full") changed dm-writecache, so that it
writes directly to the origin device if the cache is full.
Unfortunately, it doesn't forward flush requests to the origin device,
so that there is a bug where flushes are being ignored.

Fix this by adding missing flush forwarding.

For PMEM mode, we fix this bug by disabling direct writes to the origin
device, because it performs better.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: d53f1fa ("dm writecache: do direct write if the cache is full")
Cc: stable@vger.kernel.org # v5.7+
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -1297,8 +1297,12 @@ static int writecache_map(struct dm_target *ti, struct bio *bio)
 			writecache_flush(wc);
 			if (writecache_has_error(wc))
 				goto unlock_error;
+			if (unlikely(wc->cleaner))
+				goto unlock_remap_origin;
 			goto unlock_submit;
 		} else {
+			if (dm_bio_get_target_bio_nr(bio))
+				goto unlock_remap_origin;
 			writecache_offload_bio(wc, bio);
 			goto unlock_return;
 		}
@@ -1377,7 +1381,7 @@ static int writecache_map(struct dm_target *ti, struct bio *bio)
 			}
 			e = writecache_pop_from_freelist(wc, (sector_t)-1);
 			if (unlikely(!e)) {
-				if (!found_entry) {
+				if (!WC_MODE_PMEM(wc) && !found_entry) {
 direct_write:
 					e = writecache_find_entry(wc, bio->bi_iter.bi_sector, WFE_RETURN_FOLLOWING);
 					if (e) {
@@ -2484,7 +2488,7 @@ static int writecache_ctr(struct dm_target *ti, unsigned argc, char **argv)
 		goto bad;
 	}
 
-	ti->num_flush_bios = 1;
+	ti->num_flush_bios = WC_MODE_PMEM(wc) ? 1 : 2;
 	ti->flush_supported = true;
 	ti->num_discard_bios = 1;