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

[Yang Shi <shy828301@gmail.com>]

On Wed, Jan 6, 2021 at 2:22 AM Kirill Tkhai <ktkhai@virtuozzo.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@gmail.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. 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;
> >  }
> >
>
>