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

Yu Zhao
2024-11-04T22:08:47Z

On Mon, Nov 4, 2024 at 3:04 PM Yu Zhao <yuzhao@google.com> wrote:
>
> On Mon, Nov 4, 2024 at 2:38 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > 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?
>
> The assertion was caused by the patch in this thread. It used to
> assert that a folio must be protected from charge migration. Charge
> migration is removed by this series, and as part of the effort, this
> patch removes the RCU lock.
>
> > And a link to the sysbot report?
>
> https://syzkaller.appspot.com/bug?extid=24f45b8beab9788e467e

Or this link would work better: