bcache: remove unncessary code in bch_btree_keys_init()

Function bch_btree_keys_init() initializes b->set[].size and
b->set[].data to zero. As the code comments indicates, these code indeed
is unncessary, because both struct btree_keys and struct bset_tree are
nested embedded into struct btree, when struct btree is filled with 0
bits by kzalloc() in mca_bucket_alloc(), b->set[].size and
b->set[].data are initialized to 0 (a.k.a NULL) already.

This patch removes the redundant code, and add comments in
bch_btree_keys_init() and mca_bucket_alloc() to explain why it's safe.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/bset.c

@@ -347,22 +347,19 @@ EXPORT_SYMBOL(bch_btree_keys_alloc);
 void bch_btree_keys_init(struct btree_keys *b, const struct btree_keys_ops *ops,
 			 bool *expensive_debug_checks)
 {
-	unsigned int i;
-
 	b->ops = ops;
 	b->expensive_debug_checks = expensive_debug_checks;
 	b->nsets = 0;
 	b->last_set_unwritten = 0;
 
-	/* XXX: shouldn't be needed */
-	for (i = 0; i < MAX_BSETS; i++)
-		b->set[i].size = 0;
 	/*
-	 * Second loop starts at 1 because b->keys[0]->data is the memory we
-	 * allocated
+	 * struct btree_keys in embedded in struct btree, and struct
+	 * bset_tree is embedded into struct btree_keys. They are all
+	 * initialized as 0 by kzalloc() in mca_bucket_alloc(), and
+	 * b->set[0].data is allocated in bch_btree_keys_alloc(), so we
+	 * don't have to initiate b->set[].size and b->set[].data here
+	 * any more.
 	 */
-	for (i = 1; i < MAX_BSETS; i++)
-		b->set[i].data = NULL;
 }
 EXPORT_SYMBOL(bch_btree_keys_init);
 

drivers/md/bcache/btree.c

@@ -613,6 +613,10 @@ static void mca_data_alloc(struct btree *b, struct bkey *k, gfp_t gfp)
 static struct btree *mca_bucket_alloc(struct cache_set *c,
 				      struct bkey *k, gfp_t gfp)
 {
+	/*
+	 * kzalloc() is necessary here for initialization,
+	 * see code comments in bch_btree_keys_init().
+	 */
 	struct btree *b = kzalloc(sizeof(struct btree), gfp);
 
 	if (!b)