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

Suren Baghdasaryan
2021-11-09T20:02:51Z

On Tue, Nov 9, 2021 at 11:50 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 09-11-21 11:37:06, Suren Baghdasaryan wrote:
> > On Tue, Nov 9, 2021 at 11:26 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Tue 09-11-21 11:01:02, Suren Baghdasaryan wrote:
> > > [...]
> > > > Discussing how the patch I want to post works for maple trees that
> > > > Matthew is working on, I've got a question:
> > > >
> > > > IIUC, according to Michal's post here:
> > > > https://lore.kernel.org/all/20170725154514.GN26723@dhcp22.suse.cz,
> > > > unmap_vmas() can race with other mmap_lock read holders (including
> > > > oom_reap_task_mm()) with no issues.
> > > > Maple tree patchset requires rcu read lock or the mmap semaphore be
> > > > held (read or write side) when walking the tree, including inside
> > > > unmap_vmas(). When asked, he told me that he is not sure why it's
> > > > currently "safe" to walk the vma->vm_next list in unmap_vmas() while
> > > > another thread is reaping the mm.
> > > > Michal (or maybe someone else), could you please clarify why
> > > > unmap_vmas() can safely race with oom_reap_task_mm()? Or maybe my
> > > > understanding was wrong?
> > >
> > > I cannot really comment on the mapple tree part. But the existing
> > > synchronization between oom reaper and exit_mmap is based on
> > > - oom_reaper takes mmap_sem for reading
> > > - exit_mmap sets MMF_OOM_SKIP and takes the exclusive mmap_sem before
> > > unmap_vmas.
> > >
> > > The oom_reaper therefore can either unmap the address space if the lock
> > > is taken before exit_mmap or it would it would bale out on MMF_OOM_SKIP
> > > if it takes the lock afterwards. So the reaper cannot race with
> > > unmap_vmas.
> >
> > I see. So, it's the combination of MMF_OOM_SKIP and mmap_lock working
> > as a barrier which prevent them from racing with each other...
> > I wasn't sure how
> > https://lore.kernel.org/all/20170724072332.31903-1-mhocko@kernel.org/
> > was implementing this synchronization because it would take mmap_sem
> > write side after unmap_vmas() and IIUC there was no
> > "mmap_lock_write(); mmap_unlock_write();" sequence in exit_mmap at
> > that time. I'll need to checkout the old sources to figure this out.
>
> My memory is rather dimm but AFAIR the main problem was freeing page
> tables and freeing vmas not unmap_vmas. That one was no modifying the
> vma list. Essentially it was just a slightly modified madvise don't
> need. So that part was allowed to race with oom_reaper.

So, both unmap_vmas and __oom_reap_task_mm do not modify vma list and
therefore can execute concurrently. That makes sense, thanks.

Then I guess, if we want to be semantically correct in exit_mmap(), we
would have to take mmap_read_lock before unmap_vmas, then drop it and
take mmap_write_lock before free_pgtables.

> --
> Michal Hocko
> SUSE Labs