FAILED: patch "[PATCH] KVM: arm/arm64: Skip updating PMD entry if no change" failed to apply to 4.9-stable tree

FAILED: patch "[PATCH] KVM: arm/arm64: Skip updating PMD entry if no change" failed to apply to 4.9-stable tree

[gregkh]

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From 86658b819cd0a9aa584cd84453ed268a6f013770 Mon Sep 17 00:00:00 2001
From: Punit Agrawal <punit.agrawal@arm.com>
Date: Mon, 13 Aug 2018 11:43:50 +0100
Subject: [PATCH] KVM: arm/arm64: Skip updating PMD entry if no change

Contention on updating a PMD entry by a large number of vcpus can lead
to duplicate work when handling stage 2 page faults. As the page table
update follows the break-before-make requirement of the architecture,
it can lead to repeated refaults due to clearing the entry and
flushing the tlbs.

This problem is more likely when -

* there are large number of vcpus
* the mapping is large block mapping

such as when using PMD hugepages (512MB) with 64k pages.

Fix this by skipping the page table update if there is no change in
the entry being updated.

Cc: stable@vger.kernel.org
Fixes: ad361f093c1e ("KVM: ARM: Support hugetlbfs backed huge pages")
Reviewed-by: Suzuki Poulose <suzuki.poulose@arm.com>
Acked-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
index 97d27cd9c654..13dfe36501aa 100644
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c
@@ -1044,19 +1044,35 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
 	pmd = stage2_get_pmd(kvm, cache, addr);
 	VM_BUG_ON(!pmd);
 
-	/*
-	 * Mapping in huge pages should only happen through a fault.  If a
-	 * page is merged into a transparent huge page, the individual
-	 * subpages of that huge page should be unmapped through MMU
-	 * notifiers before we get here.
-	 *
-	 * Merging of CompoundPages is not supported; they should become
-	 * splitting first, unmapped, merged, and mapped back in on-demand.
-	 */
-	VM_BUG_ON(pmd_present(*pmd) && pmd_pfn(*pmd) != pmd_pfn(*new_pmd));
-
 	old_pmd = *pmd;
 	if (pmd_present(old_pmd)) {
+		/*
+		 * Multiple vcpus faulting on the same PMD entry, can
+		 * lead to them sequentially updating the PMD with the
+		 * same value. Following the break-before-make
+		 * (pmd_clear() followed by tlb_flush()) process can
+		 * hinder forward progress due to refaults generated
+		 * on missing translations.
+		 *
+		 * Skip updating the page table if the entry is
+		 * unchanged.
+		 */
+		if (pmd_val(old_pmd) == pmd_val(*new_pmd))
+			return 0;
+
+		/*
+		 * Mapping in huge pages should only happen through a
+		 * fault.  If a page is merged into a transparent huge
+		 * page, the individual subpages of that huge page
+		 * should be unmapped through MMU notifiers before we
+		 * get here.
+		 *
+		 * Merging of CompoundPages is not supported; they
+		 * should become splitting first, unmapped, merged,
+		 * and mapped back in on-demand.
+		 */
+		VM_BUG_ON(pmd_pfn(old_pmd) != pmd_pfn(*new_pmd));
+
 		pmd_clear(pmd);
 		kvm_tlb_flush_vmid_ipa(kvm, addr);
 	} else {

Re: FAILED: patch "[PATCH] KVM: arm/arm64: Skip updating PMD entry if no change" failed to apply to 4.9-stable tree

[Punit Agrawal]

Hi Greg,

<gregkh@linuxfoundation.org> writes:

> The patch below does not apply to the 4.9-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.

The patch fails to apply as the file has moved locations since
v4.9. Please find the backported patch at the end of the mail.

With the file move handled, the patch applies to v4.4 and v3.18 stable
trees without any issues as well.

Thanks,
Punit

[...]

-- >8 --
From: Punit Agrawal <punit.agrawal@arm.com>
Date: Wed, 8 Aug 2018 14:58:24 +0100
Subject: [PATCH 1/2] KVM: arm/arm64: Skip updating PMD entry if no change

commit 86658b819cd0a9aa584cd84453ed268a6f013770 upstream

Contention on updating a PMD entry by a large number of vcpus can lead
to duplicate work when handling stage 2 page faults. As the page table
update follows the break-before-make requirement of the architecture,
it can lead to repeated refaults due to clearing the entry and
flushing the tlbs.

This problem is more likely when -

* there are large number of vcpus
* the mapping is large block mapping

such as when using PMD hugepages (512MB) with 64k pages.

Fix this by skipping the page table update if there is no change in
the entry being updated.

Fixes: ad361f093c1e ("KVM: ARM: Support hugetlbfs backed huge pages")
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Reviewed-by: Suzuki Poulose <suzuki.poulose@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Christoffer Dall <christoffer.dall@arm.com>
---
 arch/arm/kvm/mmu.c | 38 +++++++++++++++++++++++++++-----------
 1 file changed, 27 insertions(+), 11 deletions(-)

diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index 7f868d9bb5ed..e3acb3a6ca37 100644
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -894,19 +894,35 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
 	pmd = stage2_get_pmd(kvm, cache, addr);
 	VM_BUG_ON(!pmd);
 
-	/*
-	 * Mapping in huge pages should only happen through a fault.  If a
-	 * page is merged into a transparent huge page, the individual
-	 * subpages of that huge page should be unmapped through MMU
-	 * notifiers before we get here.
-	 *
-	 * Merging of CompoundPages is not supported; they should become
-	 * splitting first, unmapped, merged, and mapped back in on-demand.
-	 */
-	VM_BUG_ON(pmd_present(*pmd) && pmd_pfn(*pmd) != pmd_pfn(*new_pmd));
-
 	old_pmd = *pmd;
 	if (pmd_present(old_pmd)) {
+		/*
+		 * Multiple vcpus faulting on the same PMD entry, can
+		 * lead to them sequentially updating the PMD with the
+		 * same value. Following the break-before-make
+		 * (pmd_clear() followed by tlb_flush()) process can
+		 * hinder forward progress due to refaults generated
+		 * on missing translations.
+		 *
+		 * Skip updating the page table if the entry is
+		 * unchanged.
+		 */
+		if (pmd_val(old_pmd) == pmd_val(*new_pmd))
+			return 0;
+
+		/*
+		 * Mapping in huge pages should only happen through a
+		 * fault.  If a page is merged into a transparent huge
+		 * page, the individual subpages of that huge page
+		 * should be unmapped through MMU notifiers before we
+		 * get here.
+		 *
+		 * Merging of CompoundPages is not supported; they
+		 * should become splitting first, unmapped, merged,
+		 * and mapped back in on-demand.
+		 */
+		VM_BUG_ON(pmd_pfn(old_pmd) != pmd_pfn(*new_pmd));
+
 		pmd_clear(pmd);
 		kvm_tlb_flush_vmid_ipa(kvm, addr);
 	} else {
-- 
2.18.0

Re: FAILED: patch "[PATCH] KVM: arm/arm64: Skip updating PMD entry if no change" failed to apply to 4.9-stable tree

[Greg KH]

On Fri, Aug 31, 2018 at 10:35:23AM +0100, Punit Agrawal wrote:
> Hi Greg,
> 
> <gregkh@linuxfoundation.org> writes:
> 
> > The patch below does not apply to the 4.9-stable tree.
> > If someone wants it applied there, or to any other stable or longterm
> > tree, then please email the backport, including the original git commit
> > id to <stable@vger.kernel.org>.
> 
> The patch fails to apply as the file has moved locations since
> v4.9. Please find the backported patch at the end of the mail.
> 
> With the file move handled, the patch applies to v4.4 and v3.18 stable
> trees without any issues as well.

Now queued up, along with the other backport you sent, thanks!

greg k-h