linux-kernel - Re: [PATCH v3] KVM: x86/mmu: Update number of zapped pages even if page list is stable

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Message-ID: <c21a5891-06fa-1d0b-360f-54b8711fd23b@redhat.com>
Date:   Thu, 12 May 2022 16:11:40 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v3] KVM: x86/mmu: Update number of zapped pages even if
 page list is stable

On 5/11/22 16:51, 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@...gle.com>
> Reviewed-by: Ben Gardon <bgardon@...gle.com>
> Reviewed-by: David Matlack <dmatlack@...gle.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
> 
> v3:
>   - Collect David's review.
>   - "Rebase".  The v2 patch still applies cleanly, but Paolo apparently has
>     a filter configured to ignore all emails related to the v2 submission.
> 
> v2:
>   - https://lore.kernel.org/all/20211129235233.1277558-1-seanjc@google.com
>   - Rebase to kvm/master, commit 30d7c5d60a88 ("KVM: SEV: expose...")
>   - Collect Ben's review, modulo bad splat.
>   - Copy+paste the correct splat and symptom. [David].
> 
>   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 909372762363..7429ae1784af 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5665,6 +5665,7 @@ 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,
> @@ -5696,11 +5697,12 @@ 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;
> -		}
>   	}
>   
>   	/*
> 
> base-commit: 2764011106d0436cb44702cfb0981339d68c3509

Queued, thanks.

Paolo