dm cache: boost promotion of blocks that will be overwritten

When considering whether to move a block to the cache we already give
preferential treatment to discarded blocks, since they are cheap to
promote (no read of the origin required since the data is junk).

The same is true of blocks that are about to be completely
overwritten, so we likewise boost their promotion chances.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-target.c

@@ -1697,17 +1697,17 @@ static void process_cell(struct cache *cache, struct prealloc *structs,
 	dm_oblock_t block = get_bio_block(cache, bio);
 	struct policy_result lookup_result;
 	bool passthrough = passthrough_mode(&cache->features);
-	bool discarded_block, can_migrate;
+	bool fast_promotion, can_migrate;
 	struct old_oblock_lock ool;
 
-	discarded_block = is_discarded_oblock(cache, block);
-	can_migrate = !passthrough && (discarded_block || spare_migration_bandwidth(cache));
+	fast_promotion = is_discarded_oblock(cache, block) || bio_writes_complete_block(cache, bio);
+	can_migrate = !passthrough && (fast_promotion || spare_migration_bandwidth(cache));
 
 	ool.locker.fn = cell_locker;
 	ool.cache = cache;
 	ool.structs = structs;
 	ool.cell = NULL;
-	r = policy_map(cache->policy, block, true, can_migrate, discarded_block,
+	r = policy_map(cache->policy, block, true, can_migrate, fast_promotion,
 		       bio, &ool.locker, &lookup_result);
 
 	if (r == -EWOULDBLOCK)
@@ -2895,7 +2895,7 @@ static int cache_map(struct dm_target *ti, struct bio *bio)
 	dm_oblock_t block = get_bio_block(cache, bio);
 	size_t pb_data_size = get_per_bio_data_size(cache);
 	bool can_migrate = false;
-	bool discarded_block;
+	bool fast_promotion;
 	struct policy_result lookup_result;
 	struct per_bio_data *pb = init_per_bio_data(bio, pb_data_size);
 	struct old_oblock_lock ool;
@@ -2937,9 +2937,9 @@ static int cache_map(struct dm_target *ti, struct bio *bio)
 		return DM_MAPIO_SUBMITTED;
 	}
 
-	discarded_block = is_discarded_oblock(cache, block);
+	fast_promotion = is_discarded_oblock(cache, block) || bio_writes_complete_block(cache, bio);
 
-	r = policy_map(cache->policy, block, false, can_migrate, discarded_block,
+	r = policy_map(cache->policy, block, false, can_migrate, fast_promotion,
 		       bio, &ool.locker, &lookup_result);
 	if (r == -EWOULDBLOCK) {
 		cell_defer(cache, cell, true);