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Date:   Fri, 20 Jan 2023 17:32:38 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Michal Hocko <mhocko@suse.com>, akpm@linux-foundation.org,
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Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in
 vm_area_free
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On Fri, Jan 20, 2023 at 09:17:46AM -0800, Suren Baghdasaryan wrote:
> On Fri, Jan 20, 2023 at 9:08 AM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> >
> > * Matthew Wilcox <willy@infradead.org> [230120 11:50]:
> > > On Fri, Jan 20, 2023 at 08:45:21AM -0800, Suren Baghdasaryan wrote:
> > > > On Fri, Jan 20, 2023 at 8:20 AM Suren Baghdasaryan <surenb@google.com> wrote:
> > > > >
> > > > > On Fri, Jan 20, 2023 at 12:52 AM Michal Hocko <mhocko@suse.com> wrote:
> > > > > >
> > > > > > On Thu 19-01-23 10:52:03, Suren Baghdasaryan wrote:
> > > > > > > On Thu, Jan 19, 2023 at 4:59 AM Michal Hocko <mhocko@suse.com> wrote:
> > > > > > > >
> > > > > > > > On Mon 09-01-23 12:53:34, Suren Baghdasaryan wrote:
> > > > > > > > > call_rcu() can take a long time when callback offloading is enabled.
> > > > > > > > > Its use in the vm_area_free can cause regressions in the exit path when
> > > > > > > > > multiple VMAs are being freed. To minimize that impact, place VMAs into
> > > > > > > > > a list and free them in groups using one call_rcu() call per group.
> > > > > > > >
> > > > > > > > After some more clarification I can understand how call_rcu might not be
> > > > > > > > super happy about thousands of callbacks to be invoked and I do agree
> > > > > > > > that this is not really optimal.
> > > > > > > >
> > > > > > > > On the other hand I do not like this solution much either.
> > > > > > > > VM_AREA_FREE_LIST_MAX is arbitrary and it won't really help all that
> > > > > > > > much with processes with a huge number of vmas either. It would still be
> > > > > > > > in housands of callbacks to be scheduled without a good reason.
> > > > > > > >
> > > > > > > > Instead, are there any other cases than remove_vma that need this
> > > > > > > > batching? We could easily just link all the vmas into linked list and
> > > > > > > > use a single call_rcu instead, no? This would both simplify the
> > > > > > > > implementation, remove the scaling issue as well and we do not have to
> > > > > > > > argue whether VM_AREA_FREE_LIST_MAX should be epsilon or epsilon + 1.
> > > > > > >
> > > > > > > Yes, I agree the solution is not stellar. I wanted something simple
> > > > > > > but this is probably too simple. OTOH keeping all dead vm_area_structs
> > > > > > > on the list without hooking up a shrinker (additional complexity) does
> > > > > > > not sound too appealing either.
> > > > > >
> > > > > > I suspect you have missed my idea. I do not really want to keep the list
> > > > > > around or any shrinker. It is dead simple. Collect all vmas in
> > > > > > remove_vma and then call_rcu the whole list at once after the whole list
> > > > > > (be it from exit_mmap or remove_mt). See?
> > > > >
> > > > > Yes, I understood your idea but keeping dead objects until the process
> > > > > exits even when the system is low on memory (no shrinkers attached)
> > > > > seems too wasteful. If we do this I would advocate for attaching a
> > > > > shrinker.
> > > >
> > > > Maybe even simpler, since we are hit with this VMA freeing flood
> > > > during exit_mmap (when all VMAs are destroyed), we pass a hint to
> > > > vm_area_free to batch the destruction and all other cases call
> > > > call_rcu()? I don't think there will be other cases of VMA destruction
> > > > floods.
> > >
> > > ... or have two different call_rcu functions; one for munmap() and
> > > one for exit.  It'd be nice to use kmem_cache_free_bulk().
> >
> > Do we even need a call_rcu on exit?  At the point of freeing the VMAs we
> > have set the MMF_OOM_SKIP bit and unmapped the vmas under the read lock.
> > Once we have obtained the write lock again, I think it's safe to say we
> > can just go ahead and free the VMAs directly.
> 
> I think that would be still racy if the page fault handler found that
> VMA under read-RCU protection but did not lock it yet (no locks are
> held yet). If it's preempted, the VMA can be freed and destroyed from
> under it without RCU grace period.

The page fault handler (or whatever other reader -- ptrace, proc, etc)
should have a refcount on the mm_struct, so we can't be in this path
trying to free VMAs.  Right?