dm cache: optimize dirty bit checking with find_next_bit when resizing

When shrinking the fast device, dm-cache iteratively searches for a
dirty bit among the cache blocks to be dropped, which is less efficient.
Use find_next_bit instead, as it is twice as fast as the iterative
approach with test_bit.

Signed-off-by: Ming-Hung Tsai <[email protected]>
Fixes: f494a9c ("dm cache: cache shrinking support")
Cc: [email protected]
Signed-off-by: Mikulas Patocka <[email protected]>
Acked-by: Joe Thornber <[email protected]>

Author: mingnus

drivers/md/dm-cache-target.c

@@ -2911,14 +2911,14 @@ static bool can_resize(struct cache *cache, dm_cblock_t new_size)
 	/*
 	 * We can't drop a dirty block when shrinking the cache.
 	 */
-	while (from_cblock(new_size) < from_cblock(cache->cache_size)) {
-		if (is_dirty(cache, new_size)) {
-			DMERR("%s: unable to shrink cache; cache block %llu is dirty",
-			      cache_device_name(cache),
-			      (unsigned long long) from_cblock(new_size));
-			return false;
-		}
-		new_size = to_cblock(from_cblock(new_size) + 1);
+	new_size = to_cblock(find_next_bit(cache->dirty_bitset,
+					   from_cblock(cache->cache_size),
+					   from_cblock(new_size)));
+	if (new_size != cache->cache_size) {
+		DMERR("%s: unable to shrink cache; cache block %llu is dirty",
+		      cache_device_name(cache),
+		      (unsigned long long) from_cblock(new_size));
+		return false;
 	}
 
 	return true;