Re: [PATCH v2 3/3] KVM: arm64: Mark the page dirty only if the fault is handled successfully

From: "wangyanan (Y)" <wangyanan55@huawei.com>
To: Will Deacon <will@kernel.org>
Cc: Gavin Shan <gshan@redhat.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Quentin Perret <qperret@google.com>,
	linux-kernel@vger.kernel.org, yezengruan@huawei.com,
	James Morse <james.morse@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	yuzenghui@huawei.com, wanghaibin.wang@huawei.com,
	zhukeqian1@huawei.com,
	Julien Thierry <julien.thierry.kdev@gmail.com>
Subject: Re: [PATCH v2 3/3] KVM: arm64: Mark the page dirty only if the fault is handled successfully
Date: Thu, 14 Jan 2021 17:28:04 +0800	[thread overview]
Message-ID: <6cd9c7ba-cb73-bb77-3cfc-e06b204a76d9@huawei.com> (raw)
In-Reply-To: <20210113155101.GB11892@willie-the-truck>


On 2021/1/13 23:51, Will Deacon wrote:
> On Wed, Dec 16, 2020 at 08:28:44PM +0800, Yanan Wang wrote:
>> We now mark the page dirty and set the bitmap before calling fault handlers
>> in user_mem_abort(), and we might end up having spurious dirty pages if
>> update of permissions or mapping has failed.
>> So, mark the page dirty only if the fault is handled successfully.
>>
>> Let the guest directly enter again but not return to userspace if we were
>> trying to recreate the same mapping or only change access permissions
>> with BBM, which is not permitted in the mapping path.
>>
>> Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
>> ---
>>   arch/arm64/kvm/mmu.c | 18 ++++++++++++++----
>>   1 file changed, 14 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 75814a02d189..72e516a10914 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -879,11 +879,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   	if (vma_pagesize == PAGE_SIZE && !force_pte)
>>   		vma_pagesize = transparent_hugepage_adjust(memslot, hva,
>>   							   &pfn, &fault_ipa);
>> -	if (writable) {
>> +	if (writable)
>>   		prot |= KVM_PGTABLE_PROT_W;
>> -		kvm_set_pfn_dirty(pfn);
>> -		mark_page_dirty(kvm, gfn);
>> -	}
>>   
>>   	if (fault_status != FSC_PERM && !device)
>>   		clean_dcache_guest_page(pfn, vma_pagesize);
>> @@ -911,6 +908,19 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   					     memcache);
>>   	}
>>   
>> +	/* Mark the page dirty only if the fault is handled successfully */
>> +	if (writable && !ret) {
>> +		kvm_set_pfn_dirty(pfn);
>> +		mark_page_dirty(kvm, gfn);
>> +	}
>> +
>> +	/* Let the guest directly enter again if we were trying to recreate the
>> +	 * same mapping or only change access permissions with BBM, which is not
>> +	 * permitted in the mapping path.
>> +	 */
>> +	if (ret == -EAGAIN)
>> +		ret = 0;
> Maybe just 'return ret != -EAGAIN ? ret : 0;' at the end of the function?
Yes, it's more concise.

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