Re: [v7 PATCH 07/12] mm: vmscan: use a new flag to indicate shrinker is registered

[Yang Shi <shy828301@gmail.com>]

On Tue, Feb 9, 2021 at 4:39 PM Roman Gushchin <guro@fb.com> wrote:
>
> On Tue, Feb 09, 2021 at 09:46:41AM -0800, 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.
> >
> > Acked-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > ---
> >  include/linux/shrinker.h |  7 ++++---
> >  mm/vmscan.c              | 31 +++++++++----------------------
> >  2 files changed, 13 insertions(+), 25 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 273efbf4d53c..a047980536cf 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -315,19 +315,6 @@ 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)
> > @@ -336,7 +323,7 @@ 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, shrinker, 0, 0, GFP_KERNEL);
> >       if (id < 0)
> >               goto unlock;
> >
> > @@ -499,10 +486,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
> >  {
> >       down_write(&shrinker_rwsem);
> >       list_add_tail(&shrinker->list, &shrinker_list);
> > -#ifdef CONFIG_MEMCG
> > -     if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> > -             idr_replace(&shrinker_idr, shrinker, shrinker->id);
> > -#endif
> > +     shrinker->flags |= SHRINKER_REGISTERED;
> >       up_write(&shrinker_rwsem);
> >  }
> >
> > @@ -522,13 +506,16 @@ 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);
>
> Because unregister_memcg_shrinker() will take and release shrinker_rwsem once again,
> I wonder if it's better to move it into the locked section and change the calling
> convention to require the caller to take the semaphore?

I don't think we could do that since unregister_memcg_shrinker() is
called by free_prealloced_shrinker() which is called without holding
the shrinker_rwsem by fs and workingset code.

We could add a bool parameter to indicate if the rwsem was acquired or
not, but IMHO it seems not worth it.

>
> >       kfree(shrinkrem->nr_deferred);
> >       shrinker->nr_deferred = NULL;
> >  }
> > @@ -693,7 +680,7 @@ 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;
> > --
> > 2.26.2
> >