Re: can't oom-kill zap the victim's memory?

[Michal Hocko <mhocko@kernel.org>]

On Sat 19-09-15 15:24:02, Linus Torvalds wrote:
> On Sat, Sep 19, 2015 at 8:03 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> > +
> > +static void oom_unmap_func(struct work_struct *work)
> > +{
> > +       struct mm_struct *mm = xchg(&oom_unmap_mm, NULL);
> > +
> > +       if (!atomic_inc_not_zero(&mm->mm_users))
> > +               return;
> > +
> > +       // If this is not safe we can do use_mm() + unuse_mm()
> > +       down_read(&mm->mmap_sem);
> 
> I don't think this is safe.
> 
> What makes you sure that we might not deadlock on the mmap_sem here?
> For all we know, the process that is going out of memory is in the
> middle of a mmap(), and already holds the mmap_sem for writing. No?
> 
> So at the very least that needs to be a trylock, I think.

Agreed.

> And I'm not
> sure zap_page_range() is ok with the mmap_sem only held for reading.
> Normally our rule is that you can *populate* the page tables
> concurrently, but you can't tear the down

Actually mmap_sem for reading should be sufficient because we do not
alter the layout. Both MADV_DONTNEED and MADV_FREE require read mmap_sem
for example.

-- 
Michal Hocko
SUSE Labs

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