dm writecache: do direct write if the cache is full

If the cache device is full, we do a direct write to the origin device.
Note that we must not do it if the written block is already in the cache.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -1194,6 +1194,7 @@ static int writecache_map(struct dm_target *ti, struct bio *bio)
 		}
 	} else {
 		do {
+			bool found_entry = false;
 			if (writecache_has_error(wc))
 				goto unlock_error;
 			e = writecache_find_entry(wc, bio->bi_iter.bi_sector, 0);
@@ -1204,9 +1205,21 @@ static int writecache_map(struct dm_target *ti, struct bio *bio)
 					wc->overwrote_committed = true;
 					goto bio_copy;
 				}
+				found_entry = true;
 			}
 			e = writecache_pop_from_freelist(wc, (sector_t)-1);
 			if (unlikely(!e)) {
+				if (!found_entry) {
+					e = writecache_find_entry(wc, bio->bi_iter.bi_sector, WFE_RETURN_FOLLOWING);
+					if (e) {
+						sector_t next_boundary = read_original_sector(wc, e) - bio->bi_iter.bi_sector;
+						BUG_ON(!next_boundary);
+						if (next_boundary < bio->bi_iter.bi_size >> SECTOR_SHIFT) {
+							dm_accept_partial_bio(bio, next_boundary);
+						}
+					}
+					goto unlock_remap_origin;
+				}
 				writecache_wait_on_freelist(wc);
 				continue;
 			}