bcache: add comment for cache_set->fill_iter

We have the following define for btree iterator:
	struct btree_iter {
		size_t size, used;
	#ifdef CONFIG_BCACHE_DEBUG
		struct btree_keys *b;
	#endif
		struct btree_iter_set {
			struct bkey *k, *end;
		} data[MAX_BSETS];
	};

We can see that the length of data[] field is static MAX_BSETS, which is
defined as 4 currently.

But a btree node on disk could have too many bsets for an iterator to fit
on the stack - maybe far more that MAX_BSETS. Have to dynamically allocate
space to host more btree_iter_sets.

bch_cache_set_alloc() will make sure the pool cache_set->fill_iter can
allocate an iterator equipped with enough room that can host
	(sb.bucket_size / sb.block_size)
btree_iter_sets, which is more than static MAX_BSETS.

bch_btree_node_read_done() will use that pool to allocate one iterator, to
host many bsets in one btree node.

Add more comment around cache_set->fill_iter to make code less confusing.

Signed-off-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/bcache.h

@@ -658,7 +658,11 @@ struct cache_set {
 
 	/*
 	 * A btree node on disk could have too many bsets for an iterator to fit
-	 * on the stack - have to dynamically allocate them
+	 * on the stack - have to dynamically allocate them.
+	 * bch_cache_set_alloc() will make sure the pool can allocate iterators
+	 * equipped with enough room that can host
+	 *     (sb.bucket_size / sb.block_size)
+	 * btree_iter_sets, which is more than static MAX_BSETS.
 	 */
 	mempool_t		fill_iter;
 

drivers/md/bcache/btree.c

@@ -207,6 +207,11 @@ void bch_btree_node_read_done(struct btree *b)
 	struct bset *i = btree_bset_first(b);
 	struct btree_iter *iter;
 
+	/*
+	 * c->fill_iter can allocate an iterator with more memory space
+	 * than static MAX_BSETS.
+	 * See the comment arount cache_set->fill_iter.
+	 */
 	iter = mempool_alloc(&b->c->fill_iter, GFP_NOIO);
 	iter->size = b->c->sb.bucket_size / b->c->sb.block_size;
 	iter->used = 0;