linux-kernel - Re: [PATCH] kvm: arm: Fix handling of stage2 huge mappings

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Message-ID: <4daf0783-fc50-3346-0732-1e3f53c8ccbc@arm.com>
Date:   Wed, 20 Mar 2019 10:23:39 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     marc.zyngier@....com
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        will.deacon@....com, catalin.marinas@....com, james.morse@....com,
        julien.thierry@....com, wanghaibin.wang@...wei.com,
        lious.lilei@...ilicon.com, lishuo1@...ilicon.com,
        zhengxiang9@...wei.com, yuzenghui@...wei.com,
        christoffer.dall@....com
Subject: Re: [PATCH] kvm: arm: Fix handling of stage2 huge mappings

Marc,

On 20/03/2019 10:11, Marc Zyngier wrote:
> On Wed, 20 Mar 2019 09:44:38 +0000
> Suzuki K Poulose <suzuki.poulose@....com> wrote:
> 
>> Hi Marc,
>>
>> On 20/03/2019 08:15, Marc Zyngier wrote:
>>> Hi Suzuki,
>>>
>>> On Tue, 19 Mar 2019 14:11:08 +0000,
>>> Suzuki K Poulose <suzuki.poulose@....com> wrote:
>>>>
>>>> We rely on the mmu_notifier call backs to handle the split/merge
>>>> of huge pages and thus we are guaranteed that, while creating a
>>>> block mapping, either the entire block is unmapped at stage2 or it
>>>> is missing permission.
>>>>
>>>> However, we miss a case where the block mapping is split for dirty
>>>> logging case and then could later be made block mapping, if we cancel the
>>>> dirty logging. This not only creates inconsistent TLB entries for
>>>> the pages in the the block, but also leakes the table pages for
>>>> PMD level.
>>>>
>>>> Handle this corner case for the huge mappings at stage2 by
>>>> unmapping the non-huge mapping for the block. This could potentially
>>>> release the upper level table. So we need to restart the table walk
>>>> once we unmap the range.
>>>>
>>>> Fixes : ad361f093c1e31d ("KVM: ARM: Support hugetlbfs backed huge pages")
>>>> Reported-by: Zheng Xiang <zhengxiang9@...wei.com>
>>>> Cc: Zheng Xiang <zhengxiang9@...wei.com>
>>>> Cc: Zhengui Yu <yuzenghui@...wei.com>
>>>> Cc: Marc Zyngier <marc.zyngier@....com>
>>>> Cc: Christoffer Dall <christoffer.dall@....com>
>>>> Signed-off-by: Suzuki K Poulose ...


>>>> +		if (!pmd_thp_or_huge(old_pmd)) {
>>>> +			unmap_stage2_range(kvm, addr & S2_PMD_MASK, S2_PMD_SIZE);
>>>> +			goto retry;
>>>

>>>> +		if (!stage2_pud_huge(kvm, old_pud)) {
>>>> +			unmap_stage2_range(kvm, addr & S2_PUD_MASK, S2_PUD_SIZE);
>>>

>> We should really get rid of the S2_P{U/M}D_* definitions, as they are
>> always the same as the host. The only thing that changes is the PGD size
>> which varies according to the IPA and the concatenation.
>>

Also what do you think about using  P{M,U}D_* instead of S2_P{M,U}D_*
above ? I could make that change with the respin.

> 
> Sure, feel free to send a fixed version. I'll drop the currently queued
> patch.
> 


Thanks. Sorry for the trouble.

Cheers
Suzuki