Re: [PATCH] oom_reaper: close race without using oom_lock

[Michal Hocko]

On Wed 19-07-17 05:51:03, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Tue 18-07-17 23:06:50, Tetsuo Handa wrote:
> > > Commit e2fe14564d3316d1 ("oom_reaper: close race with exiting task")
> > > guarded whole OOM reaping operations using oom_lock. But there was no
> > > need to guard whole operations. We needed to guard only setting of
> > > MMF_OOM_REAPED flag because get_page_from_freelist() in
> > > __alloc_pages_may_oom() is called with oom_lock held.
> > > 
> > > If we change to guard only setting of MMF_OOM_SKIP flag, the OOM reaper
> > > can start reaping operations as soon as wake_oom_reaper() is called.
> > > But since setting of MMF_OOM_SKIP flag at __mmput() is not guarded with
> > > oom_lock, guarding only the OOM reaper side is not sufficient.
> > > 
> > > If we change the OOM killer side to ignore MMF_OOM_SKIP flag once,
> > > there is no need to guard setting of MMF_OOM_SKIP flag, and we can
> > > guarantee a chance to call get_page_from_freelist() in
> > > __alloc_pages_may_oom() without depending on oom_lock serialization.
> > > 
> > > This patch makes MMF_OOM_SKIP act as if MMF_OOM_REAPED, and adds a new
> > > flag which acts as if MMF_OOM_SKIP, in order to close both race window
> > > (the OOM reaper side and __mmput() side) without using oom_lock.
> > 
> > Why do we need this patch when
> > http://lkml.kernel.org/r/20170626130346.26314-1-mhocko@xxxxxxxxxx
> > already removes the lock and solves another problem at once?
> 
> We haven't got an answer from Hugh and/or Andrea whether that patch is safe.

So what? I haven't see anybody disputing the correctness. And to be
honest I really dislike your patch. Yet another round kind of solutions
are just very ugly hacks usually because they are highly timing
sensitive.

> Even if that patch is safe, this patch still helps with CONFIG_MMU=n case.

Could you explain how?
-- 
Michal Hocko
SUSE Labs