Re: [Patch v3 1/9] KVM: x86/mmu: Repurpose KVM MMU shrinker to purge shadow page caches

[Mingwei Zhang]

On Tue, Jan 3, 2023 at 5:00 PM Vipin Sharma <vipinsh@xxxxxxxxxx> wrote:
>
> On Tue, Jan 3, 2023 at 11:32 AM Mingwei Zhang <mizhang@xxxxxxxxxx> wrote:
> >
> > On Wed, Dec 21, 2022 at 6:35 PM Vipin Sharma <vipinsh@xxxxxxxxxx> wrote:
> > >
> > > +static void mmu_free_sp_memory_cache(struct kvm_mmu_memory_cache *cache,
> > > +                                    spinlock_t *cache_lock)
> > > +{
> > > +       int orig_nobjs;
> > > +
> > > +       spin_lock(cache_lock);
> > > +       orig_nobjs = cache->nobjs;
> > > +       kvm_mmu_free_memory_cache(cache);
> > > +       if (orig_nobjs)
> > > +               percpu_counter_sub(&kvm_total_unused_mmu_pages, orig_nobjs);
> > > +
> > > +       spin_unlock(cache_lock);
> > > +}
> >
> > I think the mmu_cache allocation and deallocation may cause the usage
> > of GFP_ATOMIC (as observed by other reviewers as well). Adding a new
> > lock would definitely sound like a plan, but I think it might affect
> > the performance. Alternatively, I am wondering if we could use a
> > mmu_cache_sequence similar to mmu_notifier_seq to help avoid the
> > concurrency?
> >
>
> Can you explain more about the performance impact? Each vcpu will have
> its own mutex. So, only contention will be with the mmu_shrinker. This
> shrinker will use mutex_try_lock() which will not block to wait for
> the lock, it will just pass on to the next vcpu. While shrinker is
> holding the lock, vcpu will be blocked in the page fault path but I
> think it should not have a huge impact considering it will execute
> rarely and for a small time.
>
> > Similar to mmu_notifier_seq, mmu_cache_sequence should be protected by
> > mmu write lock. In the page fault path, each vcpu has to collect a
> > snapshot of  mmu_cache_sequence before calling into
> > mmu_topup_memory_caches() and check the value again when holding the
> > mmu lock. If the value is different, that means the mmu_shrinker has
> > removed the cache objects and because of that, the vcpu should retry.
> >
>
> Yeah, this can be one approach. I think it will come down to the
> performance impact of using mutex which I don't think should be a
> concern.

hmm, I think you are right that there is no performance overhead by
adding a mutex and letting the shrinker using mutex_trylock(). The
point of using a sequence counter is to avoid the new lock, since
introducing a new lock will increase management burden. So unless it
is necessary, we probably should choose a simple solution first.

In this case, I think we do have such a choice and since a similar
mechanism has already been used by mmu_notifiers.

best
-Mingwei