linux-kernel - Re: [PATCH v8 1/9] KVM: arm/arm64: Ensure only THP is candidate for adjustment

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Date:   Thu, 1 Nov 2018 09:31:21 +0100
From:   Christoffer Dall <christoffer.dall@....com>
To:     Punit Agrawal <punit.agrawal@....com>
Cc:     kvmarm@...ts.cs.columbia.edu, marc.zyngier@....com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, suzuki.poulose@....com,
        stable@...r.kernel.org
Subject: Re: [PATCH v8 1/9] KVM: arm/arm64: Ensure only THP is candidate for
 adjustment

On Wed, Oct 31, 2018 at 02:52:20PM +0000, Punit Agrawal wrote:
> Christoffer Dall <christoffer.dall@....com> writes:
> 
> > On Mon, Oct 01, 2018 at 04:54:35PM +0100, Punit Agrawal wrote:
> >> PageTransCompoundMap() returns true for hugetlbfs and THP
> >> hugepages. This behaviour incorrectly leads to stage 2 faults for
> >> unsupported hugepage sizes (e.g., 64K hugepage with 4K pages) to be
> >> treated as THP faults.
> >> 
> >> Tighten the check to filter out hugetlbfs pages. This also leads to
> >> consistently mapping all unsupported hugepage sizes as PTE level
> >> entries at stage 2.
> >> 
> >> Signed-off-by: Punit Agrawal <punit.agrawal@....com>
> >> Reviewed-by: Suzuki Poulose <suzuki.poulose@....com>
> >> Cc: Christoffer Dall <christoffer.dall@....com>
> >> Cc: Marc Zyngier <marc.zyngier@....com>
> >> Cc: stable@...r.kernel.org # v4.13+
> >
> >
> > Hmm, this function is only actually called from user_mem_abort() if we
> > have (!hugetlb), so I'm not sure the cc stable here was actually
> > warranted, nor that this patch is strictly necessary.
> >
> > It doesn't hurt, and makes the code potentially more robust for the
> > future though.
> >
> > Am I missing something?
> 
> !hugetlb is only true for hugepage sizes supported at stage 2. The
> function also got called for unsupported hugepage size at stage 2, e.g.,
> 64k hugepage with 4k page size, which then ended up doing the wrong
> thing.
> 
> Hope that adds some context. I should've added this to the commit log.
> 

To be fair you did say that this was for unsupported hugepage sizes.

Thanks for the explanation.


    Christoffer