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

Michal Hocko
2021-11-12T09:07:40Z

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.
--
Michal Hocko
SUSE Labs