KVM: MMU: Fix mmu_shrink() so that it can free mmu pages as intended
Although the possible race described in commit85b7059169
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 commit1952639665
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>
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@ -4112,17 +4112,22 @@ static int mmu_shrink(struct shrinker *shrink, struct shrink_control *sc)
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int idx;
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LIST_HEAD(invalid_list);
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/*
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* Never scan more than sc->nr_to_scan VM instances.
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* Will not hit this condition practically since we do not try
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* to shrink more than one VM and it is very unlikely to see
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* !n_used_mmu_pages so many times.
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*/
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if (!nr_to_scan--)
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break;
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/*
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* n_used_mmu_pages is accessed without holding kvm->mmu_lock
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* here. We may skip a VM instance errorneosly, but we do not
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* want to shrink a VM that only started to populate its MMU
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* anyway.
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*/
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if (kvm->arch.n_used_mmu_pages > 0) {
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if (!nr_to_scan--)
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break;
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if (!kvm->arch.n_used_mmu_pages)
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continue;
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}
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idx = srcu_read_lock(&kvm->srcu);
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spin_lock(&kvm->mmu_lock);
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