Subject: Re: [PATCH 3/3] kvm: arm/arm64: Fix locking for kvm_free_stage2_pgd
To: Suzuki K Poulose <Suzuki.Poulose@arm.com>,
        Christoffer Dall <cdall@linaro.org>
References: <1489503154-20705-1-git-send-email-suzuki.poulose@arm.com>
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Cc: linux-arm-kernel@lists.infradead.org, andreyknvl@google.com,
        dvyukov@google.com, christoffer.dall@linaro.org,
        kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
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From: Marc Zyngier <marc.zyngier@arm.com>
Organization: ARM Ltd
Message-ID: <099ce185-8d78-9870-c535-b694d60a8813@arm.com>
Date: Wed, 15 Mar 2017 15:07:19 +0000
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On 15/03/17 14:33, Suzuki K Poulose wrote:
> 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@vger.kernel.org # v3.10+
>>>>>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>>>>>> Cc: Christoffer Dall <christoffer.dall@linaro.org>
>>>>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> 
> ...
> 
>>> 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) {

Right. And should_resched() also contains a test for need_resched().

This means we can also simplify stage2_wp_range(). Awesome!

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...