Re: [REPOST] [PATCH 2/2] mm,oom: Reverse the order of setting TIF_MEMDIE and sending SIGKILL.

[Michal Hocko <mhocko@kernel.org>]

On Tue 25-08-15 23:37:27, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > > > The code would be easier then and the race window much smaller. If we
> > > > really needed to prevent from preemption then preempt_{enable,disable}
> > > > aournd the whole task_lock region + do_send_sig_info would be still
> > > > easier to follow than re-taking task_lock.
> > > 
> > > What's wrong with re-taking task_lock? It seems to me that re-taking
> > > task_lock is more straightforward and easier to follow.
> > 
> > I dunno it looks more awkward to me. You have to re-check the victim->mm
> > after retaking the lock because situation might have changed while the
> > lock was dropped. If the mark_oom_victim & do_send_sig_info are in the
> > same preempt region then nothing like that is needed. But this is
> > probably a matter of taste. I find the above more readable but let's see
> > what others think.
> 
> Disabling preemption does not guarantee that the race window is small enough.
> 
> If we set TIF_MEMDIE before sending SIGKILL, long interrupts (an extreme
> example is SysRq-t from keyboard which would last many seconds) can step
> between. We will spend some percent (the worst case is 100 percent) of memory
> reserves for allocations which are not needed for termination.

I wouldn't be worried about sysrq+t because that requires the
administrator. And IRQs shouldn't take too long normally. But I guess
you are right that this will be inherently less fragile long term. All
other callers of mark_oom_victim except for lowmem_scan are safe. Could
you update the lmk as well please?

-- 
Michal Hocko
SUSE Labs

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