linux-kernel - Re: [PATCH] mm: zswap: make the lock critical section obvious in shrink

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Message-ID: <a3fc7bc6-d9c0-43e2-a530-4dafff61203b@linux.dev>
Date: Mon, 5 Aug 2024 10:06:33 +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>,
 Takero Funaki <flintglass@...il.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: zswap: make the lock critical section obvious in
 shrink_worker()

On 2024/8/3 13:33, Yosry Ahmed wrote:
> Move the comments and spin_{lock/unlock}() calls around in
> shrink_worker() to make it obvious the lock is protecting the loop
> updating zswap_next_shrink.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>

Yeah, it's clearer.

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

Thanks.

> ---
> 
> This is intended to be squashed into "mm: zswap: fix global shrinker
> memcg iteration".
> 
> ---
>   mm/zswap.c | 14 ++++++--------
>   1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index babf0abbcc765..df620eacd1d11 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1364,24 +1364,22 @@ static void shrink_worker(struct work_struct *w)
>   	 * until the next run of shrink_worker().
>   	 */
>   	do {
> -		spin_lock(&zswap_shrink_lock);
> -
>   		/*
>   		 * Start shrinking from the next memcg after zswap_next_shrink.
>   		 * When the offline cleaner has already advanced the cursor,
>   		 * advancing the cursor here overlooks one memcg, but this
>   		 * should be negligibly rare.
> +		 *
> +		 * If we get an online memcg, keep the extra reference in case
> +		 * the original one obtained by mem_cgroup_iter() is dropped by
> +		 * zswap_memcg_offline_cleanup() while we are shrinking the
> +		 * memcg.
>   		 */
> +		spin_lock(&zswap_shrink_lock);
>   		do {
>   			memcg = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
>   			zswap_next_shrink = memcg;
>   		} while (memcg && !mem_cgroup_tryget_online(memcg));
> -		/*
> -		 * Note that if we got an online memcg, we will keep the extra
> -		 * reference in case the original reference obtained by mem_cgroup_iter
> -		 * is dropped by the zswap memcg offlining callback, ensuring that the
> -		 * memcg is not killed when we are reclaiming.
> -		 */
>   		spin_unlock(&zswap_shrink_lock);
>   
>   		if (!memcg) {