Re: [PATCH v2] KVM: x86/mmu: Update number of zapped pages even if page list is stable

[Sean Christopherson <seanjc@google.com>]

On Fri, Apr 08, 2022, Sean Christopherson wrote:
> Very high latency ping, this is still problematic and still applies cleanly.

PING!  PING!  PING!  PING!

Don't make me write a script to ping you every hour :-)

> On Mon, Nov 29, 2021, Sean Christopherson wrote:
> > When zapping obsolete pages, update the running count of zapped pages
> > regardless of whether or not the list has become unstable due to zapping
> > a shadow page with its own child shadow pages.  If the VM is backed by
> > mostly 4kb pages, KVM can zap an absurd number of SPTEs without bumping
> > the batch count and thus without yielding.  In the worst case scenario,
> > this can cause a soft lokcup.
> > 
> >  watchdog: BUG: soft lockup - CPU#12 stuck for 22s! [dirty_log_perf_:13020]
> >    RIP: 0010:workingset_activation+0x19/0x130
> >    mark_page_accessed+0x266/0x2e0
> >    kvm_set_pfn_accessed+0x31/0x40
> >    mmu_spte_clear_track_bits+0x136/0x1c0
> >    drop_spte+0x1a/0xc0
> >    mmu_page_zap_pte+0xef/0x120
> >    __kvm_mmu_prepare_zap_page+0x205/0x5e0
> >    kvm_mmu_zap_all_fast+0xd7/0x190
> >    kvm_mmu_invalidate_zap_pages_in_memslot+0xe/0x10
> >    kvm_page_track_flush_slot+0x5c/0x80
> >    kvm_arch_flush_shadow_memslot+0xe/0x10
> >    kvm_set_memslot+0x1a8/0x5d0
> >    __kvm_set_memory_region+0x337/0x590
> >    kvm_vm_ioctl+0xb08/0x1040
> > 
> > Fixes: fbb158cb88b6 ("KVM: x86/mmu: Revert "Revert "KVM: MMU: zap pages in batch""")
> > Reported-by: David Matlack <dmatlack@google.com>
> > Reviewed-by: Ben Gardon <bgardon@google.com>
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> > 
> > v2:
> >  - Rebase to kvm/master, commit 30d7c5d60a88 ("KVM: SEV: expose...")
> >  - Collect Ben's review, modulo bad splat.
> >  - Copy+paste the correct splat and symptom. [David].
> > 
> > @David, I kept the unstable declaration out of the loop, mostly because I
> > really don't like putting declarations in loops, but also because
> > nr_zapped is declared out of the loop and I didn't want to change that
> > unnecessarily or make the code inconsistent.
> > 
> >  arch/x86/kvm/mmu/mmu.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 0c839ee1282c..208c892136bf 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -5576,6 +5576,7 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
> >  {
> >  	struct kvm_mmu_page *sp, *node;
> >  	int nr_zapped, batch = 0;
> > +	bool unstable;
> >  
> >  restart:
> >  	list_for_each_entry_safe_reverse(sp, node,
> > @@ -5607,11 +5608,12 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
> >  			goto restart;
> >  		}
> >  
> > -		if (__kvm_mmu_prepare_zap_page(kvm, sp,
> > -				&kvm->arch.zapped_obsolete_pages, &nr_zapped)) {
> > -			batch += nr_zapped;
> > +		unstable = __kvm_mmu_prepare_zap_page(kvm, sp,
> > +				&kvm->arch.zapped_obsolete_pages, &nr_zapped);
> > +		batch += nr_zapped;
> > +
> > +		if (unstable)
> >  			goto restart;
> > -		}
> >  	}
> >  
> >  	/*
> > -- 
> > 2.34.0.rc2.393.gf8c9666880-goog
>