linux-kernel - Re: [PATCH] KVM: arm/arm64: Check pagesize when allocating a hugepage at Stage 2

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Date:   Thu, 11 Jan 2018 14:23:41 +0000
From:   Punit Agrawal <punit.agrawal@....com>
To:     Christoffer Dall <christoffer.dall@...aro.org>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        Marc Zyngier <marc.zyngier@....com>
Subject: Re: [PATCH] KVM: arm/arm64: Check pagesize when allocating a hugepage at Stage 2

Christoffer Dall <christoffer.dall@...aro.org> writes:

> On Thu, Jan 11, 2018 at 01:01:07PM +0000, Punit Agrawal wrote:
>> Christoffer Dall <christoffer.dall@...aro.org> writes:
>> 
>> > On Thu, Jan 04, 2018 at 06:24:33PM +0000, Punit Agrawal wrote:
>> >> KVM only supports PMD hugepages at stage 2 but doesn't actually check
>> >> that the provided hugepage memory pagesize is PMD_SIZE before populating
>> >> stage 2 entries.
>> >> 
>> >> In cases where the backing hugepage size is smaller than PMD_SIZE (such
>> >> as when using contiguous hugepages),
>> >
>> > what are contiguous hugepages and how are they created vs. a normal
>> > hugetlbfs?  Is this a kernel config thing, or how does it work?
>> 
>> Contiguous hugepages use the "Contiguous" bit (bit 52) in the page table
>> entry (pte), to mark successive entries as forming a block mapping.
>> 
>> The number of successive ptes that can be combined depend on the granule
>> size. E.g., for 4KB granule, 16 last-level ptes can form a 64KB
>> hugepage. or 16 adjacent PMD entries can form a 32MB hugepage.
>> 
>> There's no difference in instantiating contiguous hugepages vs normal
>> hugepages from a user's perspective other than passing in the
>> appropriate hugepage size.
>> 
>> There is no explicit config for contiguous hugepages - instead the
>> architectural helper to setup "hugepagesz" (see setup_hugepagesz() in
>> arch/arm64/mm/hugetlbpage.c") dictates the supported sizes.
>> 
>> Contiguous hugepage support has been enabled/disabled a few times for
>> arm64 - the latest of which is 5cd028b9d90403b ("arm64: Re-enable
>> support for contiguous hugepages").
>> 
>> >
>> >> KVM can end up creating stage 2
>> >> mappings that extend beyond the supplied memory.
>> >> 
>> >> Fix this by checking for the pagesize of userspace vma before creating
>> >> PMD hugepage at stage 2.
>> >> 
>> >> Fixes: ad361f093c1e31d ("KVM: ARM: Support hugetlbfs backed huge pages")
>> >> Signed-off-by: Punit Agrawal <punit.agrawal@....com>
>> >> Cc: Christoffer Dall <christoffer.dall@...aro.org>
>> >> Cc: Marc Zyngier <marc.zyngier@....com>
>> >> ---
>> >>  virt/kvm/arm/mmu.c | 2 +-
>> >>  1 file changed, 1 insertion(+), 1 deletion(-)
>> >> 
>> >> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>> >> index b4b69c2d1012..9dea96380339 100644
>> >> --- a/virt/kvm/arm/mmu.c
>> >> +++ b/virt/kvm/arm/mmu.c
>> >> @@ -1310,7 +1310,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>> >>  		return -EFAULT;
>> >>  	}
>> >>  
>> >> -	if (is_vm_hugetlb_page(vma) && !logging_active) {
>> >> +	if (vma_kernel_pagesize(vma) == PMD_SIZE && !logging_active) {
>> >
>> > Don't we need to also fix this in kvm_send_hwpoison_signal?
>> 
>> I think we are OK here as the signal is delivered to userspace using the
>> hva and the lsb_shift is derived from the vma as well, i.e., stage 2 is
>> not involved here.
>> 
>> Does that make sense?
>> 
>
> Yes, you're right.
>
>> >
>> > (which probably implies this will then need a backport without that for
>> > older stable kernels.  Has this been an issue from the start or did we
>> > add contiguous hugepage support at some point?)
>> 
>> I think kvm was missed out in the first (and subsequent) enabling of
>> contiguous hugepage support. The functionality didn't start out broken
>> initially.
>> 
>> Note that applying the fix as far back as it applies isn't harmful
>> though.
>> 
>
> It's a bit misleading to have the "Fixes: ad361f093c1e31d" tag, in that
> it may have people running old kernels think this could be affecting
> their workloads.  I know it's unlikely, but still.  Shouldn't the tag be
> Fixes 66b3923a1a0f "arm64: hugetlb: add support for PTE contiguous bit"
> ?
>
> That would make it a
> Cc: <stable@...r.kernel.org> # v4.5+
>

Agreed. Makes sense to go only as far back as it really matters.

Can you fix it up when applying? Or I can send a patch with an update as
well.

Thanks,
Punit

> Thanks,
> -Christoffer