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>
diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index b61b83bb..96d2213 100644
--- a/drivers/md/bcache/bcache.h
+++ b/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;
