On 06.01.2021 01:58, 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.
>
> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> ---
> include/linux/shrinker.h | 7 ++++---
> mm/vmscan.c | 13 +++++++++----
> 2 files changed, 13 insertions(+), 7 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 8da765a85569..9761c7c27412 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -494,6 +494,7 @@ 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;
In case of we introduce this new flag, we should kill old flag SHRINKER_REGISTERING,
which are not needed anymore (we should you the new flag instead of that).
> up_write(&shrinker_rwsem);
> }
>
> @@ -513,13 +514,17 @@ 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);
I do not think there are some users which registration may race with unregistration.
So, I think we should check SHRINKER_REGISTERED unlocked similar to we used to check
shrinker->nr_deferred unlocked.
> + if (!(shrinker->flags & SHRINKER_REGISTERED)) {
> + up_write(&shrinker_rwsem);
> + return;
> + }
> 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;
> }
>
