linux-kernel - Re: [PATCH RFC 21/28] mm: zswap: prevent lruvec release in zswap_folio

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Message-ID: <cf8e2dba-83dd-4ad3-b98d-2e463ea569d9@linux.dev>
Date: Fri, 18 Apr 2025 10:36:03 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Muchun Song <songmuchun@...edance.com>, hannes@...xchg.org,
 mhocko@...nel.org, roman.gushchin@...ux.dev, shakeel.butt@...ux.dev,
 muchun.song@...ux.dev, akpm@...ux-foundation.org, david@...morbit.com,
 zhengqi.arch@...edance.com, yosry.ahmed@...ux.dev, nphamcs@...il.com
Cc: linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 linux-mm@...ck.org, hamzamahfooz@...ux.microsoft.com,
 apais@...ux.microsoft.com
Subject: Re: [PATCH RFC 21/28] mm: zswap: prevent lruvec release in
 zswap_folio_swapin()

On 2025/4/15 10:45, Muchun Song wrote:
> In the near future, a folio will no longer pin its corresponding
> memory cgroup. So an lruvec returned by folio_lruvec() could be
> released without the rcu read lock or a reference to its memory
> cgroup.
> 
> In the current patch, the rcu read lock is employed to safeguard
> against the release of the lruvec in zswap_folio_swapin().
> 
> This serves as a preparatory measure for the reparenting of the
> LRU pages.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>

It should be rare to race with folio reparenting process, so
it seems ok not to "reparent" this counter "nr_disk_swapins".

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

Thanks.

> ---
>   mm/zswap.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 204fb59da33c..4a41c2371f3d 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -752,8 +752,10 @@ void zswap_folio_swapin(struct folio *folio)
>   	struct lruvec *lruvec;
>   
>   	if (folio) {
> +		rcu_read_lock();
>   		lruvec = folio_lruvec(folio);
>   		atomic_long_inc(&lruvec->zswap_lruvec_state.nr_disk_swapins);
> +		rcu_read_unlock();
>   	}
>   }
>