Re: [PATCH 3/3] kvm: arm/arm64: Fix locking for kvm_free_stage2_pgd
From: Suzuki K Poulose
Date: Wed Mar 15 2017 - 10:34:55 EST
On 15/03/17 13:28, Marc Zyngier wrote:
On 15/03/17 10:56, Christoffer Dall wrote:
On Wed, Mar 15, 2017 at 09:39:26AM +0000, Marc Zyngier wrote:
On 15/03/17 09:21, Christoffer Dall wrote:
On Tue, Mar 14, 2017 at 02:52:34PM +0000, Suzuki K Poulose wrote:
In kvm_free_stage2_pgd() we don't hold the kvm->mmu_lock while calling
unmap_stage2_range() on the entire memory range for the guest. This could
cause problems with other callers (e.g, munmap on a memslot) trying to
unmap a range.
Fixes: commit d5d8184d35c9 ("KVM: ARM: Memory virtualization setup")
Cc: stable@xxxxxxxxxxxxxxx # v3.10+
Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
Cc: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
...
ok, then there's just the concern that we may be holding a spinlock for
a very long time. I seem to recall Mario once added something where he
unlocked and gave a chance to schedule something else for each PUD or
something like that, because he ran into the issue during migration. Am
I confusing this with something else?
That definitely rings a bell: stage2_wp_range() uses that kind of trick
to give the system a chance to breathe. Maybe we could use a similar
trick in our S2 unmapping code? How about this (completely untested) patch:
diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index 962616fd4ddd..1786c24212d4 100644
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -292,8 +292,13 @@ static void unmap_stage2_range(struct kvm *kvm, phys_addr_t start, u64 size)
phys_addr_t addr = start, end = start + size;
phys_addr_t next;
+ BUG_ON(!spin_is_locked(&kvm->mmu_lock));
+
pgd = kvm->arch.pgd + stage2_pgd_index(addr);
do {
+ if (need_resched() || spin_needbreak(&kvm->mmu_lock))
+ cond_resched_lock(&kvm->mmu_lock);
nit: I think we could make the cond_resched_lock() unconditionally here:
Given, __cond_resched_lock() already does all the above checks :
kernel/sched/core.c:
int __cond_resched_lock(spinlock_t *lock)
{
int resched = should_resched(PREEMPT_LOCK_OFFSET);
...
if (spin_needbreak(lock) || resched) {
Suzuki