Re: [v7 PATCH 10/12] mm: vmscan: don't need allocate shrinker->nr_deferred for memcg aware shrinkers

[Roman Gushchin]

On Tue, Feb 09, 2021 at 09:46:44AM -0800, Yang Shi wrote:
> Now nr_deferred is available on per memcg level for memcg aware shrinkers, so don't need
> allocate shrinker->nr_deferred for such shrinkers anymore.
> 
> The prealloc_memcg_shrinker() would return -ENOSYS if !CONFIG_MEMCG or memcg is disabled
> by kernel command line, then shrinker's SHRINKER_MEMCG_AWARE flag would be cleared.
> This makes the implementation of this patch simpler.
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Reviewed-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>

Acked-by: Roman Gushchin <guro@xxxxxx>

Thanks!

> ---
>  mm/vmscan.c | 33 ++++++++++++++++++---------------
>  1 file changed, 18 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 748aa6e90f83..dfde6e7fd7f5 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -338,6 +338,9 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
>  {
>  	int id, ret = -ENOMEM;
>  
> +	if (mem_cgroup_disabled())
> +		return -ENOSYS;
> +
>  	down_write(&shrinker_rwsem);
>  	/* This may call shrinker, so it must use down_read_trylock() */
>  	id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
> @@ -417,7 +420,7 @@ static bool writeback_throttling_sane(struct scan_control *sc)
>  #else
>  static int prealloc_memcg_shrinker(struct shrinker *shrinker)
>  {
> -	return 0;
> +	return -ENOSYS;
>  }
>  
>  static void unregister_memcg_shrinker(struct shrinker *shrinker)
> @@ -528,8 +531,18 @@ unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone
>   */
>  int prealloc_shrinker(struct shrinker *shrinker)
>  {
> -	unsigned int size = sizeof(*shrinker->nr_deferred);
> +	unsigned int size;
> +	int err;
>  
> +	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> +		err = prealloc_memcg_shrinker(shrinker);
> +		if (err != -ENOSYS)
> +			return err;
> +
> +		shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
> +	}
> +
> +	size = sizeof(*shrinker->nr_deferred);
>  	if (shrinker->flags & SHRINKER_NUMA_AWARE)
>  		size *= nr_node_ids;
>  
> @@ -537,26 +550,16 @@ int prealloc_shrinker(struct shrinker *shrinker)
>  	if (!shrinker->nr_deferred)
>  		return -ENOMEM;
>  
> -	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> -		if (prealloc_memcg_shrinker(shrinker))
> -			goto free_deferred;
> -	}
>  
>  	return 0;
> -
> -free_deferred:
> -	kfree(shrinker->nr_deferred);
> -	shrinker->nr_deferred = NULL;
> -	return -ENOMEM;
>  }
>  
>  void free_prealloced_shrinker(struct shrinker *shrinker)
>  {
> -	if (!shrinker->nr_deferred)
> -		return;
> -
> -	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> +	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
>  		unregister_memcg_shrinker(shrinker);
> +		return;
> +	}
>  
>  	kfree(shrinker->nr_deferred);
>  	shrinker->nr_deferred = NULL;
> -- 
> 2.26.2
>