Re: [PATCH] mm/memcontrol: stop resize loop if limit was changed again

[Pavel Tikhomirov]

On 21/03/2024 01:09, Waiman Long wrote:
> On 3/20/24 06:03, Pavel Tikhomirov wrote:
> > In memory_max_write() we first set memcg->memory.max and only then
> > try to enforce it in loop. What if while we are in loop someone else
> > have changed memcg->memory.max but we are still trying to enforce
> > the old value? I believe this can lead to nasty consequence like getting
> > an oom on perfectly fine cgroup within it's limits or excess reclaim.
>
> Concurrent write to the same cgroup control file is not possible as the 
> underlying kernfs_open_file structure has a mutex that serialize access 
> to the file. Concurrent write to different cgroup control files is 
> possible, though.

Thanks for pointing this out, now I see it, in kernfs_fop_write_iter() 
we take of->mutex before ops->write() -> cgroup_file_write(). That means 
patch is not needed.

> Cheers,
> Longman
>
> > We also have exactly the same thing in memory_high_write().
> >
> > So let's stop enforcing old limits if we already have a new ones.
> >
> > Signed-off-by: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>
> > ---
> >   mm/memcontrol.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 61932c9215e7..81b303728491 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -6769,6 +6769,9 @@ static ssize_t memory_high_write(struct 
> > kernfs_open_file *of,
> >           unsigned long nr_pages = page_counter_read(&memcg->memory);
> >           unsigned long reclaimed;
> > +        if (memcg->memory.high != high)
> > +            break;
> > +
> >           if (nr_pages <= high)
> >               break;
> > @@ -6817,6 +6820,9 @@ static ssize_t memory_max_write(struct 
> > kernfs_open_file *of,
> >       for (;;) {
> >           unsigned long nr_pages = page_counter_read(&memcg->memory);
> > +        if (memcg->memory.max != max)
> > +            break;
> > +
> >           if (nr_pages <= max)
> >               break;

--
Best regards, Tikhomirov Pavel
Senior Software Developer, Virtuozzo.