From 35f2d16bb9ace0fb2671b8232839944ad9057c6f Mon Sep 17 00:00:00 2001 From: Takuya Yoshikawa Date: Mon, 20 Aug 2012 18:35:39 +0900 Subject: [PATCH] KVM: MMU: Fix mmu_shrink() so that it can free mmu pages as intended Although the possible race described in commit 85b7059169e128c57a3a8a3e588fb89cb2031da1 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 1952639665e92481c34c34c3e2a71bf3e66ba362 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 Cc: Gleb Natapov Signed-off-by: Avi Kivity --- arch/x86/kvm/mmu.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c index 01ca00423938..7fbd0d273ea8 100644 --- a/arch/x86/kvm/mmu.c +++ b/arch/x86/kvm/mmu.c @@ -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); -- 2.39.2