Re: [PATCH 4/9] mm: vmscan: use a new flag to indicate shrinker is registered

[Yang Shi]

On Wed, Dec 2, 2020 at 7:01 PM Roman Gushchin <guro@xxxxxx> wrote:
>
> On Wed, Dec 02, 2020 at 10:27:20AM -0800, Yang Shi wrote:
> > Currently registered shrinker is indicated by non-NULL shrinker->nr_deferred.
> > This approach is fine with nr_deferred atthe 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.
> >
> > Introduce a new "state" field to indicate if shrinker is registered or not.
> > We could use the highest bit of flags, but it may be a little bit complicated to
> > extract that bit and the flags is accessed frequently by vmscan (every time shrinker
> > is called).  So add a new field in "struct shrinker", we may waster a little bit
> > memory, but it should be very few since there should be not too many registered
> > shrinkers on a normal system.
> >
> > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> > ---
> >  include/linux/shrinker.h |  4 ++++
> >  mm/vmscan.c              | 13 +++++++++----
> >  2 files changed, 13 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> > index 0f80123650e2..0bb5be88e41d 100644
> > --- a/include/linux/shrinker.h
> > +++ b/include/linux/shrinker.h
> > @@ -35,6 +35,9 @@ struct shrink_control {
> >
> >  #define SHRINK_STOP (~0UL)
> >  #define SHRINK_EMPTY (~0UL - 1)
> > +
> > +#define SHRINKER_REGISTERED  0x1
> > +
> >  /*
> >   * A callback you can register to apply pressure to ageable caches.
> >   *
> > @@ -66,6 +69,7 @@ struct shrinker {
> >       long batch;     /* reclaim batch size, 0 = default */
> >       int seeks;      /* seeks to recreate an obj */
> >       unsigned flags;
> > +     unsigned state;
>
> Hm, can't it be another flag? It seems like we have a plenty of free bits.

I thought about this too. But I was not convinced by myself that
messing flags with state is a good practice. We may add more flags in
the future, so we may end up having something like:

flag
flag
flag
state
flag
flag
...

Maybe we could use the highest bit for state?

>
> >
> >       /* These are for internal use */
> >       struct list_head list;
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 457ce04eebf2..0d628299e55c 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -378,6 +378,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
> >       if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> >               idr_replace(&shrinker_idr, shrinker, shrinker->id);
> >  #endif
> > +     shrinker->state |= SHRINKER_REGISTERED;
> >       up_write(&shrinker_rwsem);
> >  }
> >
> > @@ -397,13 +398,17 @@ 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);
> > +     if (!shrinker->state) {
> > +             up_write(&shrinker_rwsem);
> > +             return;
> > +     }
> >       list_del(&shrinker->list);
> > +     shrinker->state &= ~SHRINKER_REGISTERED;
> >       up_write(&shrinker_rwsem);
> > +
> > +     if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> > +             unregister_memcg_shrinker(shrinker);
> >       kfree(shrinker->nr_deferred);
> >       shrinker->nr_deferred = NULL;
> >  }
> > --
> > 2.26.2
> >