From d2f96f487f471b4c718324ecd1540c89d2be52ac Mon Sep 17 00:00:00 2001 From: Shenghui Wang Date: Thu, 13 Dec 2018 22:53:46 +0800 Subject: [PATCH] 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 Signed-off-by: Coly Li Signed-off-by: Jens Axboe --- drivers/md/bcache/bcache.h | 6 +++++- drivers/md/bcache/btree.c | 5 +++++ 2 files changed, 10 insertions(+), 1 deletion(-) diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h index b61b83bbcfff..96d2213f279e 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; diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c index 3f4211b5cd33..23cb1dc7296b 100644 --- a/drivers/md/bcache/btree.c +++ b/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; -- 2.30.2