Re: [PATCH] mm: change memcg->oom_group access with atomic operations

[Martin Zhao <findns94@gmail.com>]

On Tue, Feb 21, 2023 at 4:26 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 20-02-23 23:06:24, Shakeel Butt wrote:
> > On Mon, Feb 20, 2023 at 01:09:44PM -0800, Roman Gushchin wrote:
> > > On Mon, Feb 20, 2023 at 11:16:38PM +0800, Yue Zhao wrote:
> > > > The knob for cgroup v2 memory controller: memory.oom.group
> > > > will be read and written simultaneously by user space
> > > > programs, thus we'd better change memcg->oom_group access
> > > > with atomic operations to avoid concurrency problems.
> > > >
> > > > Signed-off-by: Yue Zhao <findns94@gmail.com>
> > >
> > > Hi Yue!
> > >
> > > I'm curious, have any seen any real issues which your patch is solving?
> > > Can you, please, provide a bit more details.
> > >
> >
> > IMHO such details are not needed. oom_group is being accessed
> > concurrently and one of them can be a write access. At least
> > READ_ONCE/WRITE_ONCE is needed here. Most probably syzbot didn't
> > catch this race because it does not know about the memory.oom.group
> > interface.
>
> I do agree with Roman here. It is _always_ good to mention whether this
> is a tool/review or actual bug triggered fix. Also {READ,WRITE}_ONCE doesn't
> guarantee atomicity so it would be good to rephrase the changelog.
> Something like:
> The knob for cgroup v2 memory controller: memory.oom.group
> 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 (the knob is
> usually configured long before any workloads hits actual memcg oom)
> but it is better to use READ_ONCE/WRITE_ONCE to prevent compiler from
> doing anything funky.

Thanks a lot, I will rephrase and update my patch later.

>
> This patch is not fixing any actual user visible bug but it is in line
> of a standard practice.
> --
> Michal Hocko
> SUSE Labs