On 06.01.2021 01:58, Yang Shi wrote:
> Since memcg_shrinker_map_size just can be changd under holding shrinker_rwsem
> exclusively, the read side can be protected by holding read lock, so it sounds
> superfluous to have a dedicated mutex. This should not exacerbate the contention
> to shrinker_rwsem since just one read side critical section is added.
>
> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> ---
> mm/vmscan.c | 16 ++++++----------
> 1 file changed, 6 insertions(+), 10 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9db7b4d6d0ae..ddb9f972f856 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -187,7 +187,6 @@ static DECLARE_RWSEM(shrinker_rwsem);
> #ifdef CONFIG_MEMCG
>
> static int memcg_shrinker_map_size;
> -static DEFINE_MUTEX(memcg_shrinker_map_mutex);
>
> static void memcg_free_shrinker_map_rcu(struct rcu_head *head)
> {
> @@ -200,8 +199,6 @@ static int memcg_expand_one_shrinker_map(struct mem_cgroup *memcg,
> struct memcg_shrinker_map *new, *old;
> int nid;
>
> - lockdep_assert_held(&memcg_shrinker_map_mutex);
> -
> for_each_node(nid) {
> old = rcu_dereference_protected(
> mem_cgroup_nodeinfo(memcg, nid)->shrinker_map, true);
> @@ -250,7 +247,7 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg)
> if (mem_cgroup_is_root(memcg))
> return 0;
>
> - mutex_lock(&memcg_shrinker_map_mutex);
> + down_read(&shrinker_rwsem);
> size = memcg_shrinker_map_size;
> for_each_node(nid) {
> map = kvzalloc(sizeof(*map) + size, GFP_KERNEL);
> @@ -261,7 +258,7 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg)
> }
> rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_map, map);
Here we do STORE operation, and since we want the assignment is visible
for shrink_slab_memcg() under down_read(), we have to use down_write()
in memcg_alloc_shrinker_maps().
> }
> - mutex_unlock(&memcg_shrinker_map_mutex);
> + up_read(&shrinker_rwsem);
>
> return ret;
> }
> @@ -276,9 +273,8 @@ static int memcg_expand_shrinker_maps(int new_id)
> if (size <= old_size)
> return 0;
>
> - mutex_lock(&memcg_shrinker_map_mutex);
> if (!root_mem_cgroup)
> - goto unlock;
> + goto out;
>
> memcg = mem_cgroup_iter(NULL, NULL, NULL);
> do {
> @@ -287,13 +283,13 @@ static int memcg_expand_shrinker_maps(int new_id)
> ret = memcg_expand_one_shrinker_map(memcg, size, old_size);
> if (ret) {
> mem_cgroup_iter_break(NULL, memcg);
> - goto unlock;
> + goto out;
> }
> } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
> -unlock:
> +out:
> if (!ret)
> memcg_shrinker_map_size = size;
> - mutex_unlock(&memcg_shrinker_map_mutex);
> +
> return ret;
> }
>
>
