From: Alexandru Elisei <alexandru.elisei@arm.com>
To: Marc Zyngier <maz@kernel.org>,
linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
kvmarm@lists.cs.columbia.edu, linux-mm@kvack.org
Cc: Sean Christopherson <seanjc@google.com>,
Matthew Wilcox <willy@infradead.org>,
Paolo Bonzini <pbonzini@redhat.com>,
Will Deacon <will@kernel.org>,
Quentin Perret <qperret@google.com>,
James Morse <james.morse@arm.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
kernel-team@android.com
Subject: Re: [PATCH v2 5/6] KVM: arm64: Use get_page() instead of kvm_get_pfn()
Date: Tue, 27 Jul 2021 18:46:27 +0100 [thread overview]
Message-ID: <21cf5bb7-e70c-345b-be9e-ea009823c255@arm.com> (raw)
In-Reply-To: <20210726153552.1535838-6-maz@kernel.org>
Hi Marc,
On 7/26/21 4:35 PM, Marc Zyngier wrote:
> When mapping a THP, we are guaranteed that the page isn't reserved,
> and we can safely avoid the kvm_is_reserved_pfn() call.
>
> Replace kvm_get_pfn() with get_page(pfn_to_page()).
>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
> arch/arm64/kvm/mmu.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index ebb28dd4f2c9..b303aa143592 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -840,7 +840,7 @@ transparent_hugepage_adjust(struct kvm *kvm, struct kvm_memory_slot *memslot,
> *ipap &= PMD_MASK;
> kvm_release_pfn_clean(pfn);
> pfn &= ~(PTRS_PER_PMD - 1);
> - kvm_get_pfn(pfn);
> + get_page(pfn_to_page(pfn));
> *pfnp = pfn;
>
> return PMD_SIZE;
I am not very familiar with the mm subsystem, but I did my best to review this change.
kvm_get_pfn() uses get_page(pfn) if !PageReserved(pfn_to_page(pfn)). I looked at
the documentation for the PG_reserved page flag, and for normal memory, what
looked to me like the most probable situation where that can be set for a
transparent hugepage was for the zero page. Looked at mm/huge_memory.c, and
huge_zero_pfn is allocated via alloc_pages(__GFP_ZERO) (and other flags), which
doesn't call SetPageReserved().
I looked at how a huge page can be mapped from handle_mm_fault and from
khugepaged, and it also looks to like both are using using alloc_pages() to
allocate a new hugepage.
I also did a grep for SetPageReserved(), and there are very few places where that
is called, and none looked like they have anything to do with hugepages.
As far as I can tell, this change is correct, but I think someone who is familiar
with mm would be better suited for reviewing this patch.
next prev parent reply other threads:[~2021-07-27 17:45 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-26 15:35 [PATCH v2 0/6] KVM: Remove kvm_is_transparent_hugepage() and friends Marc Zyngier
2021-07-26 15:35 ` [PATCH v2 1/6] KVM: arm64: Introduce helper to retrieve a PTE and its level Marc Zyngier
2021-07-27 10:29 ` Quentin Perret
2021-07-27 15:25 ` Alexandru Elisei
2021-07-28 12:17 ` Marc Zyngier
2021-07-26 15:35 ` [PATCH v2 2/6] KVM: arm64: Walk userspace page tables to compute the THP mapping size Marc Zyngier
2021-07-27 15:55 ` Alexandru Elisei
2021-07-26 15:35 ` [PATCH v2 3/6] KVM: arm64: Avoid mapping size adjustment on permission fault Marc Zyngier
2021-07-27 16:00 ` Alexandru Elisei
2021-07-26 15:35 ` [PATCH v2 4/6] KVM: Remove kvm_is_transparent_hugepage() and PageTransCompoundMap() Marc Zyngier
2021-07-26 15:35 ` [PATCH v2 5/6] KVM: arm64: Use get_page() instead of kvm_get_pfn() Marc Zyngier
2021-07-27 17:46 ` Alexandru Elisei [this message]
2021-07-26 15:35 ` [PATCH v2 6/6] KVM: Get rid " Marc Zyngier
2021-08-02 13:39 ` [PATCH v2 0/6] KVM: Remove kvm_is_transparent_hugepage() and friends Marc Zyngier
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