Re: crash during oom reaper

[Michal Hocko]

On Fri 16-12-16 16:07:30, Kirill A. Shutemov wrote:
> On Fri, Dec 16, 2016 at 01:56:50PM +0100, Michal Hocko wrote:
> > On Fri 16-12-16 15:35:55, Kirill A. Shutemov wrote:
> > > On Fri, Dec 16, 2016 at 12:42:43PM +0100, Michal Hocko wrote:
> > > > On Fri 16-12-16 13:44:38, Kirill A. Shutemov wrote:
> > > > > On Fri, Dec 16, 2016 at 11:11:13AM +0100, Michal Hocko wrote:
> > > > > > On Fri 16-12-16 10:43:52, Vegard Nossum wrote:
> > > > > > [...]
> > > > > > > I don't think it's a bug in the OOM reaper itself, but either of the
> > > > > > > following two patches will fix the problem (without my understand how or
> > > > > > > why):
> > > > > > > 
> > > > > > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > > > > > > index ec9f11d4f094..37b14b2e2af4 100644
> > > > > > > --- a/mm/oom_kill.c
> > > > > > > +++ b/mm/oom_kill.c
> > > > > > > @@ -485,7 +485,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk,
> > > > > > > struct mm_struct *mm)
> > > > > > >  	 */
> > > > > > >  	mutex_lock(&oom_lock);
> > > > > > > 
> > > > > > > -	if (!down_read_trylock(&mm->mmap_sem)) {
> > > > > > > +	if (!down_write_trylock(&mm->mmap_sem)) {
> > > > > > 
> > > > > > __oom_reap_task_mm is basically the same thing as MADV_DONTNEED and that
> > > > > > doesn't require the exlusive mmap_sem. So this looks correct to me.
> > > > > 
> > > > > BTW, shouldn't we filter out all VM_SPECIAL VMAs there? Or VM_PFNMAP at
> > > > > least.
> > > > > 
> > > > > MADV_DONTNEED doesn't touch VM_PFNMAP, but I don't see anything matching
> > > > > on __oom_reap_task_mm() side.
> > > > 
> > > > I guess you are right and we should match the MADV_DONTNEED behavior
> > > > here. Care to send a patch?
> > > 
> > > Below. Testing required.
> > > 
> > > > > Other difference is that you use unmap_page_range() witch doesn't touch
> > > > > mmu_notifiers. MADV_DONTNEED goes via zap_page_range(), which invalidates
> > > > > the range. Not sure if it can make any difference here.
> > > > 
> > > > Which mmu notifier would care about this? I am not really familiar with
> > > > those users so I might miss something easily.
> > > 
> > > No idea either.
> > > 
> > > Is there any reason not to use zap_page_range here too?
> > 
> > Yes, zap_page_range is much more heavy and performs operations which
> > might lock AFAIR which I really would like to prevent from.
> 
> What exactly can block there? I don't see anything with that potential.

I would have to rememeber all the details. This is mostly off-topic for
this particular thread so I think it would be better if you could send a
full patch separatelly and we can discuss it there?
-- 
Michal Hocko
SUSE Labs