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

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Message-ID: <6uoqxdnchcsmjze4uosvdjh6ztxbvo63bs3tctjkzy47ryllwp@6wvnxdeghlru>
Date: Mon, 4 Nov 2024 14:04:11 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Yu Zhao <yuzhao@...gle.com>, 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, Nov 04, 2024 at 01:38:34PM -0800, Andrew Morton wrote:
> 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?

So, this assertion is incorrect after this patch series that has removed
the charge migration and has removed mem_cgroup_trylock_pages() / 
mem_cgroup_unlock_pages() from the caller of this function
(folio_update_gen()).