On Tue, Feb 09, 2021 at 09:46:37AM -0800, Yang Shi wrote:
> Since memcg_shrinker_map_size just can be changed under holding shrinker_rwsem
> exclusively, the read side can be protected by holding read lock, so it sounds
> superfluous to have a dedicated mutex.
>
> Kirill Tkhai suggested use write lock since:
>
> * We want the assignment to shrinker_maps is visible for shrink_slab_memcg().
> * The rcu_dereference_protected() dereferrencing in shrink_slab_memcg(), but
> in case of we use READ lock in alloc_shrinker_maps(), the dereferrencing
> is not actually protected.
> * READ lock makes alloc_shrinker_info() racy against memory allocation fail.
> alloc_shrinker_info()->free_shrinker_info() may free memory right after
> shrink_slab_memcg() dereferenced it. You may say
> shrink_slab_memcg()->mem_cgroup_online() protects us from it? Yes, sure,
> but this is not the thing we want to remember in the future, since this
> spreads modularity.
>
> And a test with heavy paging workload didn't show write lock makes things worse.
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Acked-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
Acked-by: Roman Gushchin <guro@xxxxxx>
with a small nit (below):
> ---
> mm/vmscan.c | 16 ++++++----------
> 1 file changed, 6 insertions(+), 10 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 96b08c79f18d..e4ddaaaeffe2 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 free_shrinker_map_rcu(struct rcu_head *head)
> {
> @@ -200,8 +199,6 @@ static int expand_one_shrinker_map(struct mem_cgroup *memcg,
> struct memcg_shrinker_map *new, *old;
> int nid;
>
> - lockdep_assert_held(&memcg_shrinker_map_mutex);
> -
Why not check that shrinker_rwsem is down here?
> for_each_node(nid) {
> old = rcu_dereference_protected(
> mem_cgroup_nodeinfo(memcg, nid)->shrinker_map, true);
> @@ -249,7 +246,7 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg)
> if (mem_cgroup_is_root(memcg))
> return 0;
>
> - mutex_lock(&memcg_shrinker_map_mutex);
> + down_write(&shrinker_rwsem);
> size = memcg_shrinker_map_size;
> for_each_node(nid) {
> map = kvzalloc_node(sizeof(*map) + size, GFP_KERNEL, nid);
> @@ -260,7 +257,7 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg)
> }
> rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_map, map);
> }
> - mutex_unlock(&memcg_shrinker_map_mutex);
> + up_write(&shrinker_rwsem);
>
> return ret;
> }
> @@ -275,9 +272,8 @@ static int expand_shrinker_maps(int new_id)
> if (size <= old_size)
> return 0;
>
> - mutex_lock(&memcg_shrinker_map_mutex);
And here as well. It will make the locking model more obvious and will help
to avoid errors in the future.
> if (!root_mem_cgroup)
> - goto unlock;
> + goto out;
>
> memcg = mem_cgroup_iter(NULL, NULL, NULL);
> do {
> @@ -286,13 +282,13 @@ static int expand_shrinker_maps(int new_id)
> ret = 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;
> }
>
> --
> 2.26.2
>
