dm-cache watermarks

index 7dab682..2aa97cd 100644
@@ -221,6 +221,8 @@ struct cache {
 	struct list_head completed_migrations;
 	struct list_head need_commit_migrations;
 	sector_t migration_threshold;
+	unsigned long writeback_high_watermark;
+	unsigned long writeback_low_watermark;
 	wait_queue_head_t migration_wait;
 	atomic_t nr_allocated_migrations;
 

@@ -275,6 +277,7 @@ struct cache {
 	bool commit_requested:1;
 	bool loaded_mappings:1;
 	bool loaded_discards:1;
+	bool writeback_flushing_to_low:1;
 
 	/*
 	 * Cache features such as write-through.

@@ -1433,6 +1436,29 @@ static bool spare_migration_bandwidth(struct cache *cache)
 	return current_volume < cache->migration_threshold;
 }
 
+static bool writeback_wanted(struct cache *cache)
+{
+	unsigned long dirty_pct = (atomic_read(&cache->nr_dirty) * 100) / from_cblock(cache->cache_size);
+
+	if (cache->writeback_flushing_to_low) {
+		if (dirty_pct < cache->writeback_low_watermark) {
+			cache->writeback_flushing_to_low = false;
+			return false;
+		}
+
+		return true;
+
+	} else if (dirty_pct >= cache->writeback_high_watermark) {
+		cache->writeback_flushing_to_low = true;
+		return true;
+
+	} else if (dirty_pct >= cache->writeback_low_watermark && spare_migration_bandwidth(cache)) {
+		return true;
+	}
+
+	return false;
+}
+
 static void inc_hit_counter(struct cache *cache, struct bio *bio)
 {
 	atomic_inc(bio_data_dir(bio) == READ ?

@@ -1673,7 +1699,7 @@ static void writeback_some_dirty_blocks(struct cache *cache)
 
 	memset(&structs, 0, sizeof(structs));
 
-	while (spare_migration_bandwidth(cache)) {
+	while (writeback_wanted(cache)) {
 		if (prealloc_data_structs(cache, &structs))
 			break;
 

@@ -2239,6 +2265,22 @@ static int process_config_option(struct cache *cache, const char *key, const cha
 		return 0;
 	}
 
+	if (!strcasecmp(key, "writeback_high_watermark")) {
+		if (kstrtoul(value, 10, &tmp))
+			return -EINVAL;
+
+		cache->writeback_high_watermark = tmp;
+		return 0;
+	}
+
+	if (!strcasecmp(key, "writeback_low_watermark")) {
+		if (kstrtoul(value, 10, &tmp))
+			return -EINVAL;
+
+		cache->writeback_low_watermark = tmp;
+		return 0;
+	}
+
 	return NOT_CORE_OPTION;
 }
 

@@ -2332,6 +2374,8 @@ static void set_cache_size(struct cache *cache, dm_cblock_t size)
 }
 
 #define DEFAULT_MIGRATION_THRESHOLD 2048
+#define DEFAULT_WRITEBACK_HIGH_WATERMARK 40
+#define DEFAULT_WRITEBACK_LOW_WATERMARK 30
 
 static int cache_create(struct cache_args *ca, struct cache **result)
 {

@@ -2397,6 +2441,8 @@ static int cache_create(struct cache_args *ca, struct cache **result)
 
 	cache->policy_nr_args = ca->policy_argc;
 	cache->migration_threshold = DEFAULT_MIGRATION_THRESHOLD;
+	cache->writeback_high_watermark = DEFAULT_WRITEBACK_HIGH_WATERMARK;
+	cache->writeback_low_watermark = DEFAULT_WRITEBACK_LOW_WATERMARK;
 
 	r = set_config_values(cache, ca->policy_argc, ca->policy_argv);
 	if (r) {

@@ -3103,7 +3149,7 @@ static void cache_status(struct dm_target *ti, status_type_t type,
 			goto err;
 		}
 
-		DMEMIT("2 migration_threshold %llu ", (unsigned long long) cache->migration_threshold);
+		DMEMIT("6 migration_threshold %llu writeback_high_watermark %llu writeback_low_watermark %llu ", (unsigned long long) cache->migration_threshold, (unsigned long long) cache->writeback_high_watermark, (unsigned long long) cache->writeback_low_watermark);
 
 		DMEMIT("%s ", dm_cache_policy_get_name(cache->policy));
 		if (sz < maxlen) {