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

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Message-Id: <20220511145122.3133334-1-seanjc@google.com>
Date:   Wed, 11 May 2022 14:51:22 +0000
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, David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: [PATCH v3] KVM: x86/mmu: Update number of zapped pages even if page
 list is stable

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
-- 
2.36.0.512.ge40c2bad7a-goog