Re: [PATCH v2, 3/4] mm, memcg: Prevent memory.oom_control load/store tearing

[Martin Zhao <findns94@gmail.com>]

On Tue, Mar 7, 2023 at 1:53 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 06-03-23 23:41:37, Yue Zhao wrote:
> > The knob for cgroup v1 memory controller: memory.oom_control
> > is not protected by any locking so it can be modified while it is used.
> > This is not an actual problem because races are unlikely.
> > But it is better to use READ_ONCE/WRITE_ONCE to prevent compiler from
> > doing anything funky.
> >
> > The access of memcg->oom_kill_disable is lockless,
> > so it can be concurrently set at the same time as we are
> > trying to read it.
> >
> > Signed-off-by: Yue Zhao <findns94@gmail.com>
> > ---
> >  mm/memcontrol.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index dca895c66a9b..26605b2f51b1 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -4515,7 +4515,7 @@ static int mem_cgroup_oom_control_read(struct seq_file *sf, void *v)
> >  {
> >       struct mem_cgroup *memcg = mem_cgroup_from_seq(sf);
> >
> > -     seq_printf(sf, "oom_kill_disable %d\n", memcg->oom_kill_disable);
> > +     seq_printf(sf, "oom_kill_disable %d\n", READ_ONCE(memcg->oom_kill_disable));
> >       seq_printf(sf, "under_oom %d\n", (bool)memcg->under_oom);
> >       seq_printf(sf, "oom_kill %lu\n",
> >                  atomic_long_read(&memcg->memory_events[MEMCG_OOM_KILL]));
> > @@ -4531,7 +4531,7 @@ static int mem_cgroup_oom_control_write(struct cgroup_subsys_state *css,
> >       if (mem_cgroup_is_root(memcg) || !((val == 0) || (val == 1)))
> >               return -EINVAL;
> >
> > -     memcg->oom_kill_disable = val;
> > +     WRITE_ONCE(memcg->oom_kill_disable, val);
> >       if (!val)
> >               memcg_oom_recover(memcg);
>
> Any specific reasons you haven't covered other accesses
> (mem_cgroup_css_alloc, mem_cgroup_oom, mem_cgroup_oom_synchronize)?

Thanks for point this out, you are right, we should add
[READ|WRITE]_ONCE for all used places.
Let me create PATCH v3 later.
Also for the memcg->soft_limit, I will update as well.

> >
> > --
> > 2.17.1
>
> --
> Michal Hocko
> SUSE Labs