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SubjectRe: [patch v4] mm, oom: fix unnecessary killing of additional processes
On Sat, 21 Jul 2018, Tetsuo Handa wrote:

> > diff --git a/mm/mmap.c b/mm/mmap.c
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -3066,25 +3066,27 @@ void exit_mmap(struct mm_struct *mm)
> > if (unlikely(mm_is_oom_victim(mm))) {
> > /*
> > * Manually reap the mm to free as much memory as possible.
> > - * Then, as the oom reaper does, set MMF_OOM_SKIP to disregard
> > - * this mm from further consideration. Taking mm->mmap_sem for
> > - * write after setting MMF_OOM_SKIP will guarantee that the oom
> > - * reaper will not run on this mm again after mmap_sem is
> > - * dropped.
> > - *
> > * Nothing can be holding mm->mmap_sem here and the above call
> > * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
> > * __oom_reap_task_mm() will not block.
> > *
> > + * This sets MMF_UNSTABLE to avoid racing with the oom reaper.
> > * This needs to be done before calling munlock_vma_pages_all(),
> > * which clears VM_LOCKED, otherwise the oom reaper cannot
> > - * reliably test it.
> > + * reliably test for it. If the oom reaper races with
> > + * munlock_vma_pages_all(), this can result in a kernel oops if
> > + * a pmd is zapped, for example, after follow_page_mask() has
> > + * checked pmd_none().
> > */
> > mutex_lock(&oom_lock);
> > __oom_reap_task_mm(mm);
> > mutex_unlock(&oom_lock);
>
> I don't like holding oom_lock for full teardown of an mm, for an OOM victim's mm
> might have multiple TB memory which could take long time.
>

This patch does not involve deltas for oom_lock here, it can certainly be
changed on top of this patch. I'm not attempting to address any oom_lock
issue here. It should pose no roadblock for you.

I only propose this patch now since it fixes millions of processes being
oom killed unnecessarily, it was in -mm before a NACK for the most trivial
fixes that have now been squashed into it, and is actually tested.

> >
> > - set_bit(MMF_OOM_SKIP, &mm->flags);
> > + /*
> > + * Taking mm->mmap_sem for write after setting MMF_UNSTABLE will
> > + * guarantee that the oom reaper will not run on this mm again
> > + * after mmap_sem is dropped.
> > + */
> > down_write(&mm->mmap_sem);
> > up_write(&mm->mmap_sem);
> > }
>
>
>
> > -#define MAX_OOM_REAP_RETRIES 10
> > static void oom_reap_task(struct task_struct *tsk)
> > {
> > - int attempts = 0;
> > struct mm_struct *mm = tsk->signal->oom_mm;
> >
> > - /* Retry the down_read_trylock(mmap_sem) a few times */
> > - while (attempts++ < MAX_OOM_REAP_RETRIES && !oom_reap_task_mm(tsk, mm))
> > - schedule_timeout_idle(HZ/10);
> > + /*
> > + * If this mm has either been fully unmapped, or the oom reaper has
> > + * given up on it, nothing left to do except drop the refcount.
> > + */
> > + if (test_bit(MMF_OOM_SKIP, &mm->flags))
> > + goto drop;
> >
> > - if (attempts <= MAX_OOM_REAP_RETRIES ||
> > - test_bit(MMF_OOM_SKIP, &mm->flags))
> > - goto done;
> > + /*
> > + * If this mm has already been reaped, doing so again will not likely
> > + * free additional memory.
> > + */
> > + if (!test_bit(MMF_UNSTABLE, &mm->flags))
> > + oom_reap_task_mm(tsk, mm);
>
> This is still wrong. If preempted immediately after set_bit(MMF_UNSTABLE, &mm->flags) from
> __oom_reap_task_mm() from exit_mmap(), oom_reap_task() can give up before reclaiming any memory.

If there is a single thread holding onto the mm and has reached
exit_mmap() and is in the process of starting oom reaping itself, there's
no advantage to the oom reaper trying to oom reap it. The thread in
exit_mmap() will take care of it, __oom_reap_task_mm() does not block and
oom_free_timeout_ms allows for enough time for that memory freeing to
occur. The oom reaper will not set MMF_OOM_SKIP until the timeout has
expired.

As I said before, you could make a case for extending the timeout once
MMF_UNSTABLE has been set. It practice, we haven't encountered a case
where that matters. But that's trivial to do if you would prefer.

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