linux-kernel - [PATCH] KVM: x86: Zap the oldest MMU pages, not the newest

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Date:   Wed, 13 Jan 2021 12:50:30 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        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, Zdenek Kaspar <zkaspar82@...il.com>
Subject: [PATCH] KVM: x86: Zap the oldest MMU pages, not the newest

Walk the list of MMU pages in reverse in kvm_mmu_zap_oldest_mmu_pages().
The list is FIFO, meaning new pages are inserted at the head and thus
the oldest pages are at the tail.  Using a "forward" iterator causes KVM
to zap MMU pages that were just added, which obliterates guest
performance once the max number of shadow MMU pages is reached.

Fixes: 6b82ef2c9cf1 ("KVM: x86/mmu: Batch zap MMU pages when recycling oldest pages")
Reported-by: Zdenek Kaspar <zkaspar82@...il.com>
Cc: stable@...r.kernel.org
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 6d16481aa29d..ed861245ecf0 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2417,7 +2417,7 @@ static unsigned long kvm_mmu_zap_oldest_mmu_pages(struct kvm *kvm,
 		return 0;
 
 restart:
-	list_for_each_entry_safe(sp, tmp, &kvm->arch.active_mmu_pages, link) {
+	list_for_each_entry_safe_reverse(sp, tmp, &kvm->arch.active_mmu_pages, link) {
 		/*
 		 * Don't zap active root pages, the page itself can't be freed
 		 * and zapping it will just force vCPUs to realloc and reload.
-- 
2.30.0.284.gd98b1dd5eaa7-goog