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.
>
> /* 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
>
