bcache: explicitly make cache_set only have single cache

Currently although the bcache code has a framework for multiple caches
in a cache set, but indeed the multiple caches never completed and users
use md raid1 for multiple copies of the cached data.

This patch does the following change in struct cache_set, to explicitly
make a cache_set only have single cache,
- Change pointer array "*cache[MAX_CACHES_PER_SET]" to a single pointer
  "*cache".
- Remove pointer array "*cache_by_alloc[MAX_CACHES_PER_SET]".
- Remove "caches_loaded".

Now the code looks as exactly what it does in practic: only one cache is
used in the cache set.
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/alloc.c

@@ -501,7 +501,7 @@ int __bch_bucket_alloc_set(struct cache_set *c, unsigned int reserve,
 
 	bkey_init(k);
 
-	ca = c->cache_by_alloc[0];
+	ca = c->cache;
 	b = bch_bucket_alloc(ca, reserve, wait);
 	if (b == -1)
 		goto err;

drivers/md/bcache/bcache.h

@@ -519,9 +519,7 @@ struct cache_set {
 
 	struct cache_sb		sb;
 
-	struct cache		*cache[MAX_CACHES_PER_SET];
-	struct cache		*cache_by_alloc[MAX_CACHES_PER_SET];
-	int			caches_loaded;
+	struct cache		*cache;
 
 	struct bcache_device	**devices;
 	unsigned int		devices_max_used;
@@ -808,7 +806,7 @@ static inline struct cache *PTR_CACHE(struct cache_set *c,
 				      const struct bkey *k,
 				      unsigned int ptr)
 {
-	return c->cache[PTR_DEV(k, ptr)];
+	return c->cache;
 }
 
 static inline size_t PTR_BUCKET_NR(struct cache_set *c,
@@ -890,7 +888,7 @@ do {									\
 /* Looping macros */
 
 #define for_each_cache(ca, cs, iter)					\
-	for (iter = 0; ca = cs->cache[iter], iter < (cs)->sb.nr_in_set; iter++)
+	for (iter = 0; ca = cs->cache, iter < 1; iter++)
 
 #define for_each_bucket(b, ca)						\
 	for (b = (ca)->buckets + (ca)->sb.first_bucket;			\

drivers/md/bcache/super.c

@@ -1674,7 +1674,7 @@ static void cache_set_free(struct closure *cl)
 	for_each_cache(ca, c, i)
 		if (ca) {
 			ca->set = NULL;
-			c->cache[ca->sb.nr_this_dev] = NULL;
+			c->cache = NULL;
 			kobject_put(&ca->kobj);
 		}
 
@@ -2165,7 +2165,7 @@ static const char *register_cache_set(struct cache *ca)
 
 	list_for_each_entry(c, &bch_cache_sets, list)
 		if (!memcmp(c->sb.set_uuid, ca->sb.set_uuid, 16)) {
-			if (c->cache[ca->sb.nr_this_dev])
+			if (c->cache)
 				return "duplicate cache set member";
 
 			if (!can_attach_cache(ca, c))
@@ -2215,14 +2215,11 @@ static const char *register_cache_set(struct cache *ca)
 
 	kobject_get(&ca->kobj);
 	ca->set = c;
-	ca->set->cache[ca->sb.nr_this_dev] = ca;
-	c->cache_by_alloc[c->caches_loaded++] = ca;
+	ca->set->cache = ca;
 
-	if (c->caches_loaded == c->sb.nr_in_set) {
-		err = "failed to run cache set";
-		if (run_cache_set(c) < 0)
-			goto err;
-	}
+	err = "failed to run cache set";
+	if (run_cache_set(c) < 0)
+		goto err;
 
 	return NULL;
 err:
@@ -2239,8 +2236,8 @@ void bch_cache_release(struct kobject *kobj)
 	unsigned int i;
 
 	if (ca->set) {
-		BUG_ON(ca->set->cache[ca->sb.nr_this_dev] != ca);
-		ca->set->cache[ca->sb.nr_this_dev] = NULL;
+		BUG_ON(ca->set->cache != ca);
+		ca->set->cache = NULL;
 	}
 
 	free_pages((unsigned long) ca->disk_buckets, ilog2(meta_bucket_pages(&ca->sb)));