linux-kernel - Re: [RFC PATCH 1/9] mm: zswap: always shrink in zswap_store() if zswap_pool_reached

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Date: Wed, 27 Mar 2024 10:21:08 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Yosry Ahmed <yosryahmed@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>, Nhat Pham <nphamcs@...il.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/9] mm: zswap: always shrink in zswap_store() if
 zswap_pool_reached_full

On 2024/3/26 07:50, Yosry Ahmed wrote:
> The cleanup code in zswap_store() is not pretty, particularly the
> 'shrink' label at the bottom that ends up jumping between cleanup
> labels.
> 
> Instead of having a dedicated label to shrink the pool, just use
> zswap_pool_reached_full directly to figure out if the pool needs
> shrinking. zswap_pool_reached_full should be true if and only if the
> pool needs shrinking.
> 
> The only caveat is that the value of zswap_pool_reached_full may be
> changed by concurrent zswap_store() calls between checking the limit and
> testing zswap_pool_reached_full in the cleanup code. This is fine
> because:
> - If zswap_pool_reached_full was true during limit checking then became
>   false during the cleanup code, then someone else already took care of
>   shrinking the pool and there is no need to queue the worker. That
>   would be a good change.
> - If zswap_pool_reached_full was false during limit checking then became
>   true during the cleanup code, then someone else hit the limit
>   meanwhile. In this case, both threads will try to queue the worker,
>   but it never gets queued more than once anyway. Also, calling
>   queue_work() multiple times when the limit is hit could already happen
>   today, so this isn't a significant change in any way.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>

Looks good to me.

Reviewed-by: Chengming Zhou <chengming.zhou@...ux.dev>

Thanks.

> ---
>  mm/zswap.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index c4979c76d58e3..1cf3ab4b22e64 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1429,12 +1429,12 @@ bool zswap_store(struct folio *folio)
>  	if (cur_pages >= max_pages) {
>  		zswap_pool_limit_hit++;
>  		zswap_pool_reached_full = true;
> -		goto shrink;
> +		goto reject;
>  	}
>  
>  	if (zswap_pool_reached_full) {
>  		if (cur_pages > zswap_accept_thr_pages())
> -			goto shrink;
> +			goto reject;
>  		else
>  			zswap_pool_reached_full = false;
>  	}
> @@ -1540,6 +1540,8 @@ bool zswap_store(struct folio *folio)
>  	zswap_entry_cache_free(entry);
>  reject:
>  	obj_cgroup_put(objcg);
> +	if (zswap_pool_reached_full)
> +		queue_work(shrink_wq, &zswap_shrink_work);
>  check_old:
>  	/*
>  	 * If the zswap store fails or zswap is disabled, we must invalidate the
> @@ -1550,10 +1552,6 @@ bool zswap_store(struct folio *folio)
>  	if (entry)
>  		zswap_entry_free(entry);
>  	return false;
> -
> -shrink:
> -	queue_work(shrink_wq, &zswap_shrink_work);
> -	goto reject;
>  }
>  
>  bool zswap_load(struct folio *folio)