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Date:   Tue, 15 Dec 2020 14:05:28 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     guro@...com, ktkhai@...tuozzo.com, shakeelb@...gle.com,
        hannes@...xchg.org, mhocko@...e.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 7/9] mm: vmscan: don't need allocate
 shrinker->nr_deferred for memcg aware shrinkers

On Mon, Dec 14, 2020 at 02:37:20PM -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.
> 
> Signed-off-by: Yang Shi <shy828301@...il.com>
> ---
>  mm/vmscan.c | 28 ++++++++++++++--------------
>  1 file changed, 14 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bce8cf44eca2..8d5bfd818acd 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -420,7 +420,15 @@ 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;
> +
> +	if (is_deferred_memcg_aware(shrinker)) {
> +		if (prealloc_memcg_shrinker(shrinker))
> +			return -ENOMEM;
> +		return 0;
> +	}
> +
> +	size = sizeof(*shrinker->nr_deferred);
>  
>  	if (shrinker->flags & SHRINKER_NUMA_AWARE)
>  		size *= nr_node_ids;
> @@ -429,26 +437,18 @@ 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;
>  }

I'm trying to put my finger on it, but this seems wrong to me. If
memcgs are disabled, then prealloc_memcg_shrinker() needs to fail.
The preallocation code should not care about internal memcg details
like this.

	/*
	 * If the shrinker is memcg aware and memcgs are not
	 * enabled, clear the MEMCG flag and fall back to non-memcg
	 * behaviour for the shrinker.
	 */
	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
		error = prealloc_memcg_shrinker(shrinker);
		if (!error)
			return 0;
		if (error != -ENOSYS)
			return error;

		/* memcgs not enabled! */
		shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
	}

	size = sizeof(*shrinker->nr_deferred);
	....
	return 0;
}

This guarantees that only the shrinker instances taht have a
correctly set up memcg attached to them will have the
SHRINKER_MEMCG_AWARE flag set. Hence in all the rest of the shrinker
code, we only ever need to check for SHRINKER_MEMCG_AWARE to
determine what we should do....

>  void free_prealloced_shrinker(struct shrinker *shrinker)
>  {
> -	if (!shrinker->nr_deferred)
> +	if (is_deferred_memcg_aware(shrinker)) {
> +		unregister_memcg_shrinker(shrinker);
>  		return;
> +	}
>  
> -	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> -		unregister_memcg_shrinker(shrinker);
> +	if (!shrinker->nr_deferred)
> +		return;
>  
>  	kfree(shrinker->nr_deferred);
>  	shrinker->nr_deferred = NULL;

e.g. then this function can simply do:

{
	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
		return unregister_memcg_shrinker(shrinker);
	kfree(shrinker->nr_deferred);
	shrinker->nr_deferred = NULL;
}

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com

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