Commit 35f2d16b authored Aug 20, 2012 by Takuya Yoshikawa Committed by Avi Kivity Aug 22, 2012

KVM: MMU: Fix mmu_shrink() so that it can free mmu pages as intended



Although the possible race described in

  commit 85b70591
  KVM: MMU: fix shrinking page from the empty mmu

was correct, the real cause of that issue was a more trivial bug of
mmu_shrink() introduced by

  commit 19526396
  KVM: MMU: do not iterate over all VMs in mmu_shrink()

Here is the bug:

	if (kvm->arch.n_used_mmu_pages > 0) {
		if (!nr_to_scan--)
			break;
		continue;
	}

We skip VMs whose n_used_mmu_pages is not zero and try to shrink others:
in other words we try to shrink empty ones by mistake.

This patch reverses the logic so that mmu_shrink() can free pages from
the first VM whose n_used_mmu_pages is not zero.  Note that we also add
comments explaining the role of nr_to_scan which is not practically
important now, hoping this will be improved in the future.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Cc: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

parent e8143ccb

arch/x86/kvm/mmu.c

+9 −4

Original line number	Diff line number	Diff line
		@@ -4112,17 +4112,22 @@ static int mmu_shrink(struct shrinker shrink, struct shrink_control sc)
		int idx;
		LIST_HEAD(invalid_list);

		/*
		* Never scan more than sc->nr_to_scan VM instances.
		* Will not hit this condition practically since we do not try
		* to shrink more than one VM and it is very unlikely to see
		* !n_used_mmu_pages so many times.
		*/
		if (!nr_to_scan--)
		break;
		/*
		* n_used_mmu_pages is accessed without holding kvm->mmu_lock
		* here. We may skip a VM instance errorneosly, but we do not
		* want to shrink a VM that only started to populate its MMU
		* anyway.
		*/
		if (kvm->arch.n_used_mmu_pages > 0) {
		if (!nr_to_scan--)
		break;
		if (!kvm->arch.n_used_mmu_pages)
		continue;
		}

		idx = srcu_read_lock(&kvm->srcu);
		spin_lock(&kvm->mmu_lock);

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