linux-kernel - Re: [PATCH v1 5/6] memcg-v1: no need for memcg locking for MGLRU

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Message-Id: <20241104133834.e0e138038a111c2b0d20bdde@linux-foundation.org>
Date: Mon, 4 Nov 2024 13:38:34 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Yu Zhao <yuzhao@...gle.com>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>, Johannes Weiner
 <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, Roman Gushchin
 <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, Hugh
 Dickins <hughd@...gle.com>, Yosry Ahmed <yosryahmed@...gle.com>,
 linux-mm@...ck.org, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org, Meta kernel team
 <kernel-team@...a.com>
Subject: Re: [PATCH v1 5/6] memcg-v1: no need for memcg locking for MGLRU

On Mon, 4 Nov 2024 10:30:29 -0700 Yu Zhao <yuzhao@...gle.com> wrote:

> On Sat, Oct 26, 2024 at 09:26:04AM -0600, Yu Zhao wrote:
> > On Sat, Oct 26, 2024 at 12:34 AM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> > >
> > > On Thu, Oct 24, 2024 at 06:23:02PM GMT, Shakeel Butt wrote:
> > > > While updating the generation of the folios, MGLRU requires that the
> > > > folio's memcg association remains stable. With the charge migration
> > > > deprecated, there is no need for MGLRU to acquire locks to keep the
> > > > folio and memcg association stable.
> > > >
> > > > Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> > >
> > > Andrew, can you please apply the following fix to this patch after your
> > > unused fixup?
> > 
> > Thanks!
> 
> syzbot caught the following:
> 
>   WARNING: CPU: 0 PID: 85 at mm/vmscan.c:3140 folio_update_gen+0x23d/0x250 mm/vmscan.c:3140
>   ...
> 
> Andrew, can you please fix this in place?

OK, but...

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3138,7 +3138,6 @@ static int folio_update_gen(struct folio *folio, int gen)
>  	unsigned long new_flags, old_flags = READ_ONCE(folio->flags);
>  
>  	VM_WARN_ON_ONCE(gen >= MAX_NR_GENS);
> -	VM_WARN_ON_ONCE(!rcu_read_lock_held());
>  
>  	do {
>  		/* lru_gen_del_folio() has isolated this page? */

it would be good to know why this assertion is considered incorrect? 
And a link to the sysbot report?

Thanks.