Re: [v5 PATCH 06/11] mm: vmscan: use a new flag to indicate shrinker is registered

[Yang Shi <shy828301@gmail.com>]

On Thu, Jan 28, 2021 at 9:56 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 1/28/21 12:33 AM, 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.
> >
> > Remove SHRINKER_REGISTERING since we could check if shrinker is registered
> > successfully by the new flag.
> >
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > ---
> >  include/linux/shrinker.h |  7 ++++---
> >  mm/vmscan.c              | 27 +++++++++------------------
> >  2 files changed, 13 insertions(+), 21 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 92e917033797..256896d157d4 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -308,19 +308,6 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
> >       }
> >  }
> >
> > -/*
> > - * We allow subsystems to populate their shrinker-related
> > - * LRU lists before register_shrinker_prepared() is called
> > - * for the shrinker, since we don't want to impose
> > - * restrictions on their internal registration order.
> > - * In this case shrink_slab_memcg() may find corresponding
> > - * bit is set in the shrinkers map.
> > - *
> > - * This value is used by the function to detect registering
> > - * shrinkers and to skip do_shrink_slab() calls for them.
> > - */
> > -#define SHRINKER_REGISTERING ((struct shrinker *)~0UL)
> > -
> >  static DEFINE_IDR(shrinker_idr);
> >
> >  static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> > @@ -329,7 +316,7 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> >
> >       down_write(&shrinker_rwsem);
> >       /* This may call shrinker, so it must use down_read_trylock() */
> > -     id = idr_alloc(&shrinker_idr, SHRINKER_REGISTERING, 0, 0, GFP_KERNEL);
> > +     id = idr_alloc(&shrinker_idr, NULL, 0, 0, GFP_KERNEL);
>
> Is it still needed to register a NULL and then replace it with real pointer,
> given the SHRINKER_REGISTERED flag?

Good question. Should not need this alloc-replace sequence anymore.
The shrinker_slab_memcg() should see SHRINKER_REGISTERED set when the
shrinker is really registered since the registration is protected by
write shrinker_rwsem.

Will fix this in v6.

>
> >       if (id < 0)
> >               goto unlock;
> >
> > @@ -496,6 +483,7 @@ 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;
> >       up_write(&shrinker_rwsem);
> >  }
> >
> > @@ -515,13 +503,16 @@ EXPORT_SYMBOL(register_shrinker);
> >   */
> >  void unregister_shrinker(struct shrinker *shrinker)
> >  {
> > -     if (!shrinker->nr_deferred)
> > +     if (!(shrinker->flags & SHRINKER_REGISTERED))
> >               return;
> > -     if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> > -             unregister_memcg_shrinker(shrinker);
> > +
> >       down_write(&shrinker_rwsem);
> >       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;
> >  }
> > @@ -687,7 +678,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
> >               struct shrinker *shrinker;
> >
> >               shrinker = idr_find(&shrinker_idr, i);
> > -             if (unlikely(!shrinker || shrinker == SHRINKER_REGISTERING)) {
> > +             if (unlikely(!shrinker || !(shrinker->flags & SHRINKER_REGISTERED))) {
> >                       if (!shrinker)
> >                               clear_bit(i, info->map);
> >                       continue;
> >
>