Re: [PATCH] memcg: Don't call schedule_work when no spinning is allowed

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Aug 31, 2026 at 06:04:57PM -0700, David Stevens wrote:
> On Mon, Aug 31, 2026 at 5:10 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > On Mon, Aug 31, 2026 at 04:43:39PM -0700, David Stevens wrote:
> > > Memcg charging can be done from any context, but calling schedule_work()
> > > isn't safe from an NMI. If memory.high is breached from a context where
> > > spinning isn't allowed, use irq_work to schedule the reclaim work.
> > >
> > > Fixes: 3ac4638a734a ("memcg: make memcg_rstat_updated nmi safe")
> > > Signed-off-by: David Stevens <stevensd@google.com>
> >
> > Did you hit this issue or just code inspection? I assume this is the
> > done_restock code path.
> 
> I just found this via code inspection. I spent a little bit trying to
> trigger it for real, but the only way I managed was by writing a hacky
> driver absuing alloc_pages_nolock().
> 
> > > ---
> > >  include/linux/memcontrol.h |  1 +
> > >  mm/memcontrol.c            | 12 +++++++++++-
> > >  2 files changed, 12 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > > index 8170bb8066a2..036d973ceca6 100644
> > > --- a/include/linux/memcontrol.h
> > > +++ b/include/linux/memcontrol.h
> > > @@ -219,6 +219,7 @@ struct mem_cgroup {
> > >       spinlock_t       peaks_lock;
> > >
> > >       /* Range enforcement for interrupt charges */
> > > +     struct irq_work high_irq_work;
> >
> > Instead of adding more complexity, let's just return if we can not spin on
> > done_restock path.
> >
> 
> There would be no guarantee that memcg reclaim would ever be
> triggered, which also would also stop MEMCG_HIGH events from being
> generated. Overall that seems a more serious than just dropping
> userspace notifications like is done for MEMCG_MAX.

I'm leaning that way too. It's an indefinite error, and it's a freely
programmable surface.

IMO, a few lines of relatively straight-forward, self-explanatory code
is better than code that needs a comment and leaves a problem that
somebody in the future might run into.