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
> >
