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

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Oct 29, 2021 at 6:03 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 27-10-21 09:08:21, Suren Baghdasaryan wrote:
> > On Fri, Oct 22, 2021 at 10:38 AM Suren Baghdasaryan <surenb@google.com> wrote:
> > >
> > > On Fri, Oct 22, 2021 at 1:03 AM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > On Thu 21-10-21 18:46:58, Suren Baghdasaryan wrote:
> > > > > Race between process_mrelease and exit_mmap, where free_pgtables is
> > > > > called while __oom_reap_task_mm is in progress, leads to kernel crash
> > > > > during pte_offset_map_lock call. oom-reaper avoids this race by setting
> > > > > MMF_OOM_VICTIM flag and causing exit_mmap to take and release
> > > > > mmap_write_lock, blocking it until oom-reaper releases mmap_read_lock.
> > > > > Reusing MMF_OOM_VICTIM for process_mrelease would be the simplest way to
> > > > > fix this race, however that would be considered a hack. Fix this race
> > > > > by elevating mm->mm_users and preventing exit_mmap from executing until
> > > > > process_mrelease is finished. Patch slightly refactors the code to adapt
> > > > > for a possible mmget_not_zero failure.
> > > > > This fix has considerable negative impact on process_mrelease performance
> > > > > and will likely need later optimization.
> > > >
> > > > I am not sure there is any promise that process_mrelease will run in
> > > > parallel with the exiting process. In fact the primary purpose of this
> > > > syscall is to provide a reliable way to oom kill from user space. If you
> > > > want to optimize process exit resp. its exit_mmap part then you should
> > > > be using other means. So I would be careful calling this a regression.
> > > >
> > > > I do agree that taking the reference count is the right approach here. I
> > > > was wrong previously [1] when saying that pinning the mm struct is
> > > > sufficient. I have completely forgot about the subtle sync in exit_mmap.
> > > > One way we can approach that would be to take exclusive mmap_sem
> > > > throughout the exit_mmap unconditionally.
> > >
> > > I agree, that would probably be the cleanest way.
> > >
> > > > There was a push back against
> > > > that though so arguments would have to be re-evaluated.
> > >
> > > I'll review that discussion to better understand the reasons for the
> > > push back. Thanks for the link.
> >
> > Adding Kirill and Andrea.
> >
> > I had some time to dig some more. The latency increase is definitely
> > coming due to process_mrelease calling the last mmput and exit_aio is
> > especially problematic. So, currently process_mrelease not only
> > releases memory but does more, including waiting for io to finish.
>
> Well, I still do not see why that is a problem. This syscall is meant to
> release the address space not to do it fast.

It's the same problem for a userspace memory reaper as for the
oom-reaper. The goal is to release the memory of the victim and to
quickly move on to the next one if needed.

>
> > Unconditional mmap_write_lock around free_pgtables in exit_mmap seems
> > to me the most semantically correct way forward and the pushback is on
> > the basis of regressing performance of the exit path. I would like to
> > measure that regression to confirm this. I don't have access to a big
> > machine but will ask someone in another Google team to try the test
> > Michal wrote here
> > https://lore.kernel.org/all/20170725142626.GJ26723@dhcp22.suse.cz/ on
> > a server with and without a custom patch.
>
> Well, I do not remember all the details of the discussion but I believe
> a rather large part of that discussion was a bit misled. The exist
> path - and the last mmput in particular - shouldn't trigger mmap_sem
> contention. There are only rare cases where somebody can race and take a
> lock then (e.g. proc interfaces taking the lock before mmget_notzero).
> Certainly not something to optimize for and I believe a correct and
> robust code should have a preference. As we can see a lack of proper
> synchronization has led to 2 very similar problem nobody revealed during
> review because the code is just too tricky.

I totally agree that this locking is tricky and mmap_sem contention
should be very rare in the exit_mmap path and not worth optimizing.

>
> Btw. the above code will not really tell you much on a larger machine
> unless you manage to trigger mmap_sem contection. Otherwise you are
> measuring the mmap_sem writelock fast path and that should be really
> within a noise comparing to the whole address space destruction time. If
> that is not the case then we have a real problem with the locking...

My understanding of that discussion is that the concern was that even
taking uncontended mmap_sem writelock would regress the exit path.
That was what I wanted to confirm. Am I misreading it?
Thanks,
Suren.

> --
> Michal Hocko
> SUSE Labs