dm thin: prefetch missing metadata pages

Prefetch metadata at the start of the worker thread and then again every
128th bio processed from the deferred list.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-thin-metadata.c

@@ -1809,3 +1809,8 @@ bool dm_pool_metadata_needs_check(struct dm_pool_metadata *pmd)
 
 	return needs_check;
 }
+
+void dm_pool_issue_prefetches(struct dm_pool_metadata *pmd)
+{
+	dm_tm_issue_prefetches(pmd->tm);
+}

drivers/md/dm-thin-metadata.h

@@ -213,6 +213,11 @@ int dm_pool_register_metadata_threshold(struct dm_pool_metadata *pmd,
 int dm_pool_metadata_set_needs_check(struct dm_pool_metadata *pmd);
 bool dm_pool_metadata_needs_check(struct dm_pool_metadata *pmd);
 
+/*
+ * Issue any prefetches that may be useful.
+ */
+void dm_pool_issue_prefetches(struct dm_pool_metadata *pmd);
+
 /*----------------------------------------------------------------*/
 
 #endif

drivers/md/dm-thin.c

@@ -1526,6 +1526,7 @@ static void process_thin_deferred_bios(struct thin_c *tc)
 	struct bio *bio;
 	struct bio_list bios;
 	struct blk_plug plug;
+	unsigned count = 0;
 
 	if (tc->requeue_mode) {
 		requeue_bio_list(tc, &tc->deferred_bio_list);
@@ -1567,6 +1568,10 @@ static void process_thin_deferred_bios(struct thin_c *tc)
 			pool->process_discard(tc, bio);
 		else
 			pool->process_bio(tc, bio);
+
+		if ((count++ & 127) == 0) {
+			dm_pool_issue_prefetches(pool->pmd);
+		}
 	}
 	blk_finish_plug(&plug);
 }
@@ -1652,6 +1657,7 @@ static void do_worker(struct work_struct *ws)
 {
 	struct pool *pool = container_of(ws, struct pool, worker);
 
+	dm_pool_issue_prefetches(pool->pmd);
 	process_prepared(pool, &pool->prepared_mappings, &pool->process_prepared_mapping);
 	process_prepared(pool, &pool->prepared_discards, &pool->process_prepared_discard);
 	process_deferred_bios(pool);
@@ -1996,10 +2002,6 @@ static int thin_bio_map(struct dm_target *ti, struct bio *bio)
 		/* fall through */
 
 	case -EWOULDBLOCK:
-		/*
-		 * In future, the failed dm_thin_find_block above could
-		 * provide the hint to load the metadata into cache.
-		 */
 		thin_defer_bio(tc, bio);
 		cell_defer_no_holder_no_free(tc, &cell1);
 		return DM_MAPIO_SUBMITTED;