Re: [patch v2] mm, oom: fix concurrent munlock and oom reaperunmap

Michal Hocko
2018-04-24T20:13:56Z

On Tue 24-04-18 13:01:03, David Rientjes wrote:
> On Tue, 24 Apr 2018, Michal Hocko wrote:
>
> > Is there any reason why we cannot simply call __oom_reap_task_mm as we
> > have it now? mmap_sem for read shouldn't fail here because this is the
> > last reference of the mm and we are past the ksm and khugepaged
> > synchronizations. So unless my jed laged brain fools me the patch should
> > be as simple as the following (I haven't tested it at all).
> >
>
> I wanted to remove all per task checks because they are now irrelevant:
> this would be the first dependency that exit_mmap() has on any
> task_struct, which isn't intuitive -- we simply want to exit the mmap.
> There's no requirement that current owns the mm other than this.

There is no such requirement in the __oom_reap_task_mm. The given task
is used for reporting purposes.

> I wanted
> to avoid the implicit dependency on MMF_OOM_SKIP and make it explicit in
> the exit path to be matched with the oom reaper.

Well, I find it actually better that the code is not explicit about
MMF_OOM_SKIP. The whole thing happens in the oom proper which should be
really preferable. The whole synchronization is then completely
transparent to the oom (including the oom lock etc).

> I didn't want anything
> additional printed to the kernel log about oom reaping unless the
> oom_reaper actually needed to intervene, which is useful knowledge outside
> of basic exiting.

Can we shave all those parts as follow ups and make the fix as simple as
possible?

> My patch has passed intensive testing on both x86 and powerpc, so I'll ask
> that it's pushed for 4.17-rc3. Many thanks to Tetsuo for the suggestion
> on calling __oom_reap_task_mm() from exit_mmap().

Yeah, but your patch does have a problem with blockable mmu notifiers
IIUC.
--
Michal Hocko
SUSE Labs