Re: [v3 PATCH 05/11] mm: vmscan: use a new flag to indicate shrinker is registered

[Kirill Tkhai <ktkhai@virtuozzo.com>]

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@gmail.com>
> ---
>  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;
>  }
>