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| author | Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp> | 2012-08-20 09:35:39 (GMT) |
|---|---|---|
| committer | Avi Kivity <avi@redhat.com> | 2012-08-22 12:27:13 (GMT) |
| commit | 35f2d16bb9ace0fb2671b8232839944ad9057c6f (patch) | |
| tree | 763610b668cb62664b55a838488e96dad829a9b0 /arch/x86/kvm | |
| parent | e8143ccb6b501f78bb95d9c5ee100d18423008cf (diff) | |
| download | linux-35f2d16bb9ace0fb2671b8232839944ad9057c6f.tar.xz | |
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 <yoshikawa.takuya@oss.ntt.co.jp>
Cc: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Diffstat (limited to 'arch/x86/kvm')
| -rw-r--r-- | arch/x86/kvm/mmu.c | 13 |
1 files changed, 9 insertions, 4 deletions
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c index 01ca004..7fbd0d2 100644 --- a/arch/x86/kvm/mmu.c +++ b/arch/x86/kvm/mmu.c @@ -4113,16 +4113,21 @@ static int mmu_shrink(struct shrinker *shrink, struct shrink_control *sc) 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); |