linux-kernel - Re: [v3 PATCH 05/11] mm: vmscan: use a new flag to indicate shrinker is registered

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Date:   Tue, 12 Jan 2021 12:58:10 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     Roman Gushchin <guro@...com>, Shakeel Butt <shakeelb@...gle.com>,
        Dave Chinner <david@...morbit.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: [v3 PATCH 05/11] mm: vmscan: use a new flag to indicate shrinker
 is registered

On Mon, Jan 11, 2021 at 1:38 PM Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
>
> On 11.01.2021 21:17, Yang Shi wrote:
> > On Wed, Jan 6, 2021 at 2:22 AM Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
> >>
> >> On 06.01.2021 01:58, Yang Shi wrote:
> >>> Currently registered shrinker is indicated by non-NULL shrinker->nr_deferred.
> >>> This approach is fine with nr_deferred at the shrinker level, but the following
> >>> patches will move MEMCG_AWARE shrinkers' nr_deferred to memcg level, so their
> >>> shrinker->nr_deferred would always be NULL.  This would prevent the shrinkers
> >>> from unregistering correctly.
> >>>
> >>> Signed-off-by: Yang Shi <shy828301@...il.com>
> >>> ---
> >>>  include/linux/shrinker.h |  7 ++++---
> >>>  mm/vmscan.c              | 13 +++++++++----
> >>>  2 files changed, 13 insertions(+), 7 deletions(-)
> >>>
> >>> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> >>> index 0f80123650e2..1eac79ce57d4 100644
> >>> --- a/include/linux/shrinker.h
> >>> +++ b/include/linux/shrinker.h
> >>> @@ -79,13 +79,14 @@ struct shrinker {
> >>>  #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
> >>>
> >>>  /* Flags */
> >>> -#define SHRINKER_NUMA_AWARE  (1 << 0)
> >>> -#define SHRINKER_MEMCG_AWARE (1 << 1)
> >>> +#define SHRINKER_REGISTERED  (1 << 0)
> >>> +#define SHRINKER_NUMA_AWARE  (1 << 1)
> >>> +#define SHRINKER_MEMCG_AWARE (1 << 2)
> >>>  /*
> >>>   * It just makes sense when the shrinker is also MEMCG_AWARE for now,
> >>>   * non-MEMCG_AWARE shrinker should not have this flag set.
> >>>   */
> >>> -#define SHRINKER_NONSLAB     (1 << 2)
> >>> +#define SHRINKER_NONSLAB     (1 << 3)
> >>>
> >>>  extern int prealloc_shrinker(struct shrinker *shrinker);
> >>>  extern void register_shrinker_prepared(struct shrinker *shrinker);
> >>> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >>> index 8da765a85569..9761c7c27412 100644
> >>> --- a/mm/vmscan.c
> >>> +++ b/mm/vmscan.c
> >>> @@ -494,6 +494,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
> >>>       if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> >>>               idr_replace(&shrinker_idr, shrinker, shrinker->id);
> >>>  #endif
> >>> +     shrinker->flags |= SHRINKER_REGISTERED;
> >>
> >> In case of we introduce this new flag, we should kill old flag SHRINKER_REGISTERING,
> >> which are not needed anymore (we should you the new flag instead of that).
> >
> > The only think that I'm confused with is the check in
> > shrink_slab_memcg, it does:
> >
> > shrinker = idr_find(&shrinker_idr, i);
> > if (unlikely(!shrinker || shrinker == SHRINKER_REGISTERING)) {
> >
> > When allocating idr, the shrinker is associated with
> > SHRINKER_REGISTERING. But, shrink_slab_memcg does acquire read
> > shrinker_rwsem, and idr_alloc is called with holding write
> > shrinker_rwsem, so I'm supposed shrink_slab_memcg should never see
> > shrinker is registering.
>
> After prealloc_shrinker() shrinker is visible for shrink_slab_memcg().
> This is the moment shrink_slab_memcg() sees SHRINKER_REGISTERED.

Yes, this exactly is what I'm supposed.

>
> > If so it seems easy to remove
> > SHRINKER_REGISTERING.
> >
> > We just need change that check to:
> > !shrinker || !(shrinker->flags & SHRINKER_REGISTERED)
> >
> >>>       up_write(&shrinker_rwsem);
> >>>  }
> >>>
> >>> @@ -513,13 +514,17 @@ EXPORT_SYMBOL(register_shrinker);
> >>>   */
> >>>  void unregister_shrinker(struct shrinker *shrinker)
> >>>  {
> >>> -     if (!shrinker->nr_deferred)
> >>> -             return;
> >>> -     if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> >>> -             unregister_memcg_shrinker(shrinker);
> >>>       down_write(&shrinker_rwsem);
> >>
> >> I do not think there are some users which registration may race with unregistration.
> >> So, I think we should check SHRINKER_REGISTERED unlocked similar to we used to check
> >> shrinker->nr_deferred unlocked.
> >
> > Yes, I agree.
> >
> >>
> >>> +     if (!(shrinker->flags & SHRINKER_REGISTERED)) {
> >>> +             up_write(&shrinker_rwsem);
> >>> +             return;
> >>> +     }
> >>>       list_del(&shrinker->list);
> >>> +     shrinker->flags &= ~SHRINKER_REGISTERED;
> >>>       up_write(&shrinker_rwsem);
> >>> +
> >>> +     if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> >>> +             unregister_memcg_shrinker(shrinker);
> >>>       kfree(shrinker->nr_deferred);
> >>>       shrinker->nr_deferred = NULL;
> >>>  }
> >>>
> >>
> >>
>
>