On Thu 02-06-16 15:11:08, Michal Hocko wrote:
> On Thu 02-06-16 21:20:03, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > On Wed 01-06-16 15:53:13, Andrew Morton wrote:
> [...]
> > > > Is it even possible to hit that race?
> > >
> > > It is, we can have a concurrent mmput followed by mmdrop.
> > >
> > > > find_lock_task_mm() takes some
> > > > care to prevent a NULL ->mm. But I guess a concurrent mmput() doesn't
> > > > require task_lock(). Kinda makes me wonder what's the point in even
> > > > having find_lock_task_mm() if its guarantee on ->mm is useless...
> > >
> > > find_lock_task_mm makes sure that the mm stays non-NULL while we hold
> > > the lock. We have to do all the necessary pinning while holding it.
> > > atomic_inc_not_zero will guarantee we are not racing with the finall
> > > mmput.
> > >
> > > Does that make more sense now?
> >
> > what Andrew wanted to confirm is "how can it be possible that
> > mm->mm_users < 1 when there is a tsk with tsk->mm != NULL", isn't it?
> >
> > Indeed, find_lock_task_mm() returns a tsk where tsk->mm != NULL with
> > tsk->alloc_lock held. Therefore, tsk->mm != NULL implies mm->mm_users > 0
> > until we release tsk->alloc_lock , and we can do
> >
> > p = find_lock_task_mm(tsk);
> > if (!p)
> > goto unlock_oom;
> >
> > mm = p->mm;
> > - if (!atomic_inc_not_zero(&mm->mm_users)) {
> > - task_unlock(p);
> > - goto unlock_oom;
> > - }
> > + atomic_inc(&mm->mm_users);
> >
> > task_unlock(p);
> >
> > in __oom_reap_task() (unless I'm missing something).
>
> OK, I guess you are right. Care to send a patch?
I led it rest overnight and realized on the way to work this morning
that this is a left over from the earlier approach when mm_reaper got
mm rather than a task. We used to pin mm_count in oom_kill_process so
we had to do atomic_inc_not_zero on mm_users here. Now that we used
find_lock_task_mm we indeed can simply increase mm_users while holding
the lock.
Thanks!
--
Michal Hocko
SUSE Labs
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