Re: [PATCH mm-unstable v3 4/5] mm: restart if multiple traversals raced

[Yu Zhao <yuzhao@google.com>]

On Fri, Aug 30, 2024 at 11:45 AM Kinsey Ho <kinseyho@google.com> wrote:
>
> On Fri, Aug 30, 2024 at 3:04 AM Hugh Dickins <hughd@google.com> wrote:
> >
> > mm-unstable commit 954dd0848c61 needs the fix below to be merged in;
> > but the commit after it (the 5/5) then renames "memcg" to "next",
> > so that one has to be adjusted too.
> >
> > [PATCH] mm: restart if multiple traversals raced: fix
> >
> > mem_cgroup_iter() reset memcg to NULL before the goto restart, so that
> > goto out_unlock does not then return an ungotten memcg, causing oopses
> > on stale memcg in many places (often in memcg_rstat_updated()).
> >
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > ---
> >  mm/memcontrol.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 6f66ac0ad4f0..dd82dd1e1f0a 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1049,6 +1049,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> >                 if (cmpxchg(&iter->position, pos, memcg) != pos) {
> >                         if (css && css != &root->css)
> >                                 css_put(css);
> > +                       memcg = NULL;
> >                         goto restart;
> >                 }
> >
> > --
> > 2.35.3
>
> Hi Andrew,
>
> Would you prefer that I resend the series with Hugh's fix inserted?

Please send a new version to get this properly fixed, preferably move
the initialization of `memcg` from the declaration to right below
`restart`, and also add the following footers:

Reported-by: syzbot+e099d407346c45275ce9@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/000000000000817cf10620e20d33@google.com/