Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap

Suren Baghdasaryan
2021-11-12T16:00:19Z

On Fri, Nov 12, 2021 at 12:58 AM 'Michal Hocko' via kernel-team
<kernel-team@android.com> wrote:
>
> On Thu 11-11-21 07:02:42, Suren Baghdasaryan wrote:
> > On Thu, Nov 11, 2021 at 1:20 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Wed 10-11-21 17:49:37, Suren Baghdasaryan wrote:
> > > > On Tue, Nov 9, 2021 at 1:10 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > > > >
> > > > > On Tue, Nov 9, 2021 at 12:10 PM Michal Hocko <mhocko@suse.com> wrote:
> > > [...]
> > > > > > Yes, those can run concurrently. One thing I completely forgot about is
> > > > > > 27ae357fa82b ("mm, oom: fix concurrent munlock and oom reaper unmap, v3")
> > > > > > which is about interaction with the munlock.
> > > >
> > > > Agrh! This interaction with the munlock you mentioned requires us to
> > > > take mmap_write_lock before munlock_vma_pages_all and that prevents
> > > > __oom_reap_task_mm from running concurrently with unmap_vmas. The
> > > > reapers would not be as effective as they are now after such a change
> > > > :(
> > >
> > > __oom_reap_task_mm will not run concurrently with unmap_vmas even
> > > with the current code. The mmap_sem barrier right before munlock code
> > > prevents that.
> >
> > You are right, it will run concurrently with another
> > __oom_reap_task_mm in the exit_mmap. But I thought we wanted to get
> > rid of that call to __oom_reap_task_mm in exit_mmap or did I
> > misunderstand?
>
> I do not remember this to be objective or the motivation. IIRC we wanted
> to make the locking more robust which would help your process_mrelease
> use case. This one currently suffers from a much heavier cost if it
> turns out to be the last holder of the reference count on the address
> space.

Ok, I wrongly assumed the mmap_lock cleanup should be deeper. Will
keep pounding on it. Thanks!

> --
> Michal Hocko
> SUSE Labs
>
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