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

Andrew Morton
2024-11-04T21:38:35Z

On Mon, 4 Nov 2024 10:30:29 -0700 Yu Zhao <yuzhao@google.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@linux.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@linux.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.