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

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Date:   Wed, 20 Mar 2019 10:35:11 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Suzuki K Poulose <suzuki.poulose@....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

On Wed, 20 Mar 2019 10:23:39 +0000
Suzuki K Poulose <suzuki.poulose@....com> wrote:

Hi Suzuki,

> 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.

Given that this is a fix, I'd like it to be as small as obvious as
possible, making it easier to backport.

I'm happy to take another patch for 5.2 that will drop the whole S2_P*
if we still think that this should be the case (though what I'd really
like is to have architectural levels instead of these arbitrary
definitions).

Thanks,

	M.
-- 
Without deviation from the norm, progress is not possible.