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    SubjectRe: [PATCH 5/9] mm: memcontrol: add per memcg shrinker nr_deferred
    On Thu, Dec 10, 2020 at 7:36 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
    >
    > 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.

    The concurrent shrinkers registration may have race. But, they should
    get different IDs, so it seems not matter.

    I agree it is a little bit confusing and not that straightforward, it
    does owe some comments in the code.

    >
    > 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)?

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