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Date:   Thu, 17 Dec 2020 16:56:48 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     Roman Gushchin <guro@...com>, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [v2 PATCH 7/9] mm: vmscan: don't need allocate
 shrinker->nr_deferred for memcg aware shrinkers

On Tue, Dec 15, 2020 at 3:07 PM Yang Shi <shy828301@...il.com> wrote:
>
> On Mon, Dec 14, 2020 at 7:05 PM Dave Chinner <david@...morbit.com> wrote:
> >
> > On Mon, Dec 14, 2020 at 02:37:20PM -0800, Yang Shi wrote:
> > > Now nr_deferred is available on per memcg level for memcg aware shrinkers, so don't need
> > > allocate shrinker->nr_deferred for such shrinkers anymore.
> > >
> > > Signed-off-by: Yang Shi <shy828301@...il.com>
> > > ---
> > >  mm/vmscan.c | 28 ++++++++++++++--------------
> > >  1 file changed, 14 insertions(+), 14 deletions(-)
> > >
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index bce8cf44eca2..8d5bfd818acd 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -420,7 +420,15 @@ unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone
> > >   */
> > >  int prealloc_shrinker(struct shrinker *shrinker)
> > >  {
> > > -     unsigned int size = sizeof(*shrinker->nr_deferred);
> > > +     unsigned int size;
> > > +
> > > +     if (is_deferred_memcg_aware(shrinker)) {
> > > +             if (prealloc_memcg_shrinker(shrinker))
> > > +                     return -ENOMEM;
> > > +             return 0;
> > > +     }
> > > +
> > > +     size = sizeof(*shrinker->nr_deferred);
> > >
> > >       if (shrinker->flags & SHRINKER_NUMA_AWARE)
> > >               size *= nr_node_ids;
> > > @@ -429,26 +437,18 @@ int prealloc_shrinker(struct shrinker *shrinker)
> > >       if (!shrinker->nr_deferred)
> > >               return -ENOMEM;
> > >
> > > -     if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> > > -             if (prealloc_memcg_shrinker(shrinker))
> > > -                     goto free_deferred;
> > > -     }
> > > -
> > >       return 0;
> > > -
> > > -free_deferred:
> > > -     kfree(shrinker->nr_deferred);
> > > -     shrinker->nr_deferred = NULL;
> > > -     return -ENOMEM;
> > >  }
> >
> > I'm trying to put my finger on it, but this seems wrong to me. If
> > memcgs are disabled, then prealloc_memcg_shrinker() needs to fail.
> > The preallocation code should not care about internal memcg details
> > like this.
> >
> >         /*
> >          * If the shrinker is memcg aware and memcgs are not
> >          * enabled, clear the MEMCG flag and fall back to non-memcg
> >          * behaviour for the shrinker.
> >          */
> >         if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> >                 error = prealloc_memcg_shrinker(shrinker);
> >                 if (!error)
> >                         return 0;
> >                 if (error != -ENOSYS)
> >                         return error;
> >
> >                 /* memcgs not enabled! */
> >                 shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
> >         }
> >
> >         size = sizeof(*shrinker->nr_deferred);
> >         ....
> >         return 0;
> > }
> >
> > This guarantees that only the shrinker instances taht have a
> > correctly set up memcg attached to them will have the
> > SHRINKER_MEMCG_AWARE flag set. Hence in all the rest of the shrinker
> > code, we only ever need to check for SHRINKER_MEMCG_AWARE to
> > determine what we should do....
>
> Thanks. I see your point. We could move the memcg specific details
> into prealloc_memcg_shrinker().
>
> It seems we have to acquire shrinker_rwsem before we check and modify
> SHIRNKER_MEMCG_AWARE bit if we may clear it.

Hi Dave,

Is it possible that shrinker register races with shrinker unregister?
It seems impossible to me by a quick visual code inspection. But I'm
not a VFS expert so I'm not quite sure.

If it is impossible the implementation would be quite simple otherwise
we need move shrinker_rwsem acquire/release to
prealloc_shrinker/free_prealloced_shrinker/unregister_shrinker to
protect SHRINKER_MEMCG_AWARE update.

>
> >
> > >  void free_prealloced_shrinker(struct shrinker *shrinker)
> > >  {
> > > -     if (!shrinker->nr_deferred)
> > > +     if (is_deferred_memcg_aware(shrinker)) {
> > > +             unregister_memcg_shrinker(shrinker);
> > >               return;
> > > +     }
> > >
> > > -     if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> > > -             unregister_memcg_shrinker(shrinker);
> > > +     if (!shrinker->nr_deferred)
> > > +             return;
> > >
> > >       kfree(shrinker->nr_deferred);
> > >       shrinker->nr_deferred = NULL;
> >
> > e.g. then this function can simply do:
> >
> > {
> >         if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> >                 return unregister_memcg_shrinker(shrinker);
> >         kfree(shrinker->nr_deferred);
> >         shrinker->nr_deferred = NULL;
> > }
> >
> > Cheers,
> >
> > Dave.
> > --
> > Dave Chinner
> > david@...morbit.com

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