On Wed, Dec 02, 2020 at 10:27:21AM -0800, Yang Shi wrote:
> @@ -504,6 +577,34 @@ int memcg_expand_shrinker_maps(int new_id)
> return ret;
> }
>
> +int memcg_expand_shrinker_deferred(int new_id)
> +{
> + int size, old_size, ret = 0;
> + struct mem_cgroup *memcg;
> +
> + size = (new_id + 1) * sizeof(atomic_long_t);
> + old_size = memcg_shrinker_deferred_size;
> + if (size <= old_size)
> + return 0;
> +
> + mutex_lock(&memcg_shrinker_mutex);
The locking is somewhat confusing. I was wondering why we first read
memcg_shrinker_deferred_size "locklessly", then change it while
holding the &memcg_shrinker_mutex.
memcg_shrinker_deferred_size only changes under shrinker_rwsem(write),
correct? This should be documented in a comment, IMO.
memcg_shrinker_mutex looks superfluous then. The memcg allocation path
is the read-side of memcg_shrinker_deferred_size, and so simply needs
to take shrinker_rwsem(read) to lock out shrinker (de)registration.
Also, isn't memcg_shrinker_deferred_size just shrinker_nr_max? And
memcg_expand_shrinker_deferred() is only called when size >= old_size
in the first place (because id >= shrinker_nr_max)?
