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

Although the possible race described in

  commit 85b7059169
  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 1952639665
  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>
This commit is contained in:
Takuya Yoshikawa 2012-08-20 18:35:39 +09:00 committed by Avi Kivity
parent e8143ccb6b
commit 35f2d16bb9
1 changed files with 9 additions and 4 deletions
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13
arch/x86/kvm/mmu.c
View File

@ -4112,17 +4112,22 @@ static int mmu_shrink(struct shrinker *shrink, struct shrink_control *sc)
int idx; int idx;
LIST_HEAD(invalid_list); 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 * n_used_mmu_pages is accessed without holding kvm->mmu_lock
* here. We may skip a VM instance errorneosly, but we do not * here. We may skip a VM instance errorneosly, but we do not
* want to shrink a VM that only started to populate its MMU * want to shrink a VM that only started to populate its MMU
* anyway. * anyway.
*/ */
if (kvm->arch.n_used_mmu_pages > 0) { if (!kvm->arch.n_used_mmu_pages)
if (!nr_to_scan--)
break;
continue; continue;
}
idx = srcu_read_lock(&kvm->srcu); idx = srcu_read_lock(&kvm->srcu);
spin_lock(&kvm->mmu_lock); spin_lock(&kvm->mmu_lock);