On 10/24/18 10:35 AM, Shenghui Wang wrote:
> 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@xxxxxxxxxxx>
Thanks, added to my for-next.
Coly Li
> ---
> 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;
>
