Re: [PATCH v4 3/3] mm/oom_kill: allow process_mrelease to run under mmap_lock protection

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Dec 9, 2021 at 12:59 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 08-12-21 13:22:11, Suren Baghdasaryan wrote:
> > With exit_mmap holding mmap_write_lock during free_pgtables call,
> > process_mrelease does not need to elevate mm->mm_users in order to
> > prevent exit_mmap from destrying pagetables while __oom_reap_task_mm
> > is walking the VMA tree. The change prevents process_mrelease from
> > calling the last mmput, which can lead to waiting for IO completion
> > in exit_aio.
> >
> > Fixes: 337546e83fc7 ("mm/oom_kill.c: prevent a race between process_mrelease and exit_mmap")
>
> I am not sure I have brought this up already but I do not think Fixes
> tag is a good fit. 337546e83fc7 is a correct way to handle the race. It
> is just slightly less optimal than this fix.

Will post v5 without it. Thanks!

>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>
> Acked-by: Michal Hocko <mhocko@suse.com>
>
> Thanks!
> > ---
> >  mm/oom_kill.c | 27 +++++++++++++++------------
> >  1 file changed, 15 insertions(+), 12 deletions(-)
> >
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index 1ddabefcfb5a..67780386f478 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -1169,15 +1169,15 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
> >               goto put_task;
> >       }
> >
> > -     if (mmget_not_zero(p->mm)) {
> > -             mm = p->mm;
> > -             if (task_will_free_mem(p))
> > -                     reap = true;
> > -             else {
> > -                     /* Error only if the work has not been done already */
> > -                     if (!test_bit(MMF_OOM_SKIP, &mm->flags))
> > -                             ret = -EINVAL;
> > -             }
> > +     mm = p->mm;
> > +     mmgrab(mm);
> > +
> > +     if (task_will_free_mem(p))
> > +             reap = true;
> > +     else {
> > +             /* Error only if the work has not been done already */
> > +             if (!test_bit(MMF_OOM_SKIP, &mm->flags))
> > +                     ret = -EINVAL;
> >       }
> >       task_unlock(p);
> >
> > @@ -1188,13 +1188,16 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
> >               ret = -EINTR;
> >               goto drop_mm;
> >       }
> > -     if (!__oom_reap_task_mm(mm))
> > +     /*
> > +      * Check MMF_OOM_SKIP again under mmap_read_lock protection to ensure
> > +      * possible change in exit_mmap is seen
> > +      */
> > +     if (!test_bit(MMF_OOM_SKIP, &mm->flags) && !__oom_reap_task_mm(mm))
> >               ret = -EAGAIN;
> >       mmap_read_unlock(mm);
> >
> >  drop_mm:
> > -     if (mm)
> > -             mmput(mm);
> > +     mmdrop(mm);
> >  put_task:
> >       put_task_struct(task);
> >       return ret;
> > --
> > 2.34.1.400.ga245620fadb-goog
>
> --
> Michal Hocko
> SUSE Labs