Re: [patch 01/12] mm: oom_kill: remove unnecessary locking in oom_enable()

Michal Hocko
2015-03-26T13:18:42Z

On Thu 26-03-15 12:51:40, Michal Hocko wrote:
> On Wed 25-03-15 17:51:31, David Rientjes wrote:
> > On Wed, 25 Mar 2015, Johannes Weiner wrote:
> >
> > > Setting oom_killer_disabled to false is atomic, there is no need for
> > > further synchronization with ongoing allocations trying to OOM-kill.
> > >
> > > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > > ---
> > > mm/oom_kill.c | 2 --
> > > 1 file changed, 2 deletions(-)
> > >
> > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > > index 2b665da1b3c9..73763e489e86 100644
> > > --- a/mm/oom_kill.c
> > > +++ b/mm/oom_kill.c
> > > @@ -488,9 +488,7 @@ bool oom_killer_disable(void)
> > > */
> > > void oom_killer_enable(void)
> > > {
> > > - down_write(&oom_sem);
> > > oom_killer_disabled = false;
> > > - up_write(&oom_sem);
> > > }
> > >
> > > #define K(x) ((x) << (PAGE_SHIFT-10))
> >
> > I haven't looked through the new disable-oom-killer-for-pm patchset that
> > was merged, but this oom_killer_disabled thing already looks improperly
> > handled. I think any correctness or cleanups in this area would be very
> > helpful.
> >
> > I think mark_tsk_oom_victim() in mem_cgroup_out_of_memory() is just
> > luckily not racing with a call to oom_killer_enable() and triggering the
> ^^^^^^^^^^
> oom_killer_disable?
>
> > WARN_ON(oom_killer_disabled) since there's no "oom_sem" held here, and
> > it's an improper context based on the comment of mark_tsk_oom_victim().
>
> OOM killer is disabled only _after_ all user tasks have been frozen. So
> we cannot get any page fault and a race. So the semaphore is not needed
> in this path although the comment says otherwise. I can add a comment
> clarifying this...

I am wrong here! pagefault_out_of_memory takes the lock and so the whole
mem_cgroup_out_of_memory is called under the same lock.
--
Michal Hocko
SUSE Labs