linux-kernel - Re: [PATCH v2 2/7] KVM: arm64: MTE: Update code comments

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Message-ID: <Z4Ecbi9QUScEstCQ@arm.com>
Date: Fri, 10 Jan 2025 13:11:10 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: "Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev, Suzuki K Poulose <Suzuki.Poulose@....com>,
	Steven Price <steven.price@....com>, Will Deacon <will@...nel.org>,
	Marc Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Joey Gouly <joey.gouly@....com>, Zenghui Yu <yuzenghui@...wei.com>
Subject: Re: [PATCH v2 2/7] KVM: arm64: MTE: Update code comments

On Fri, Jan 10, 2025 at 04:30:18PM +0530, Aneesh Kumar K.V (Arm) wrote:
> commit d77e59a8fccd ("arm64: mte: Lock a page for MTE tag
> initialisation") updated the locking such the kernel now allows
> VM_SHARED mapping with MTE. Update the code comment to reflect this.
> 
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@...nel.org>
> ---
>  arch/arm64/kvm/mmu.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index c9d46ad57e52..eb8220a409e1 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1391,11 +1391,11 @@ static int get_vma_page_shift(struct vm_area_struct *vma, unsigned long hva)
>   * able to see the page's tags and therefore they must be initialised first. If
>   * PG_mte_tagged is set, tags have already been initialised.
>   *
> - * The race in the test/set of the PG_mte_tagged flag is handled by:
> - * - preventing VM_SHARED mappings in a memslot with MTE preventing two VMs
> - *   racing to santise the same page
> - * - mmap_lock protects between a VM faulting a page in and the VMM performing
> - *   an mprotect() to add VM_MTE
> + * The race in the test/set of the PG_mte_tagged flag is handled by using
> + * PG_mte_lock and PG_mte_tagged together. if PG_mte_lock is found unset, we can
> + * go ahead and clear the page tags. if PG_mte_lock is found set, then the page
> + * tags are already cleared or there is a parallel tag clearing is going on. We
				  ^^^^^^^^
				  remove this (or the other 'is')


> + * wait for the parallel tag clear to finish by waiting on PG_mte_tagged bit.
>   */

I don't think we need to describe the behaviour of set_page_mte_tagged()
and try_page_mte_tagging() in here. How the locking works for tagged
pages was hidden in those functions with their own documentation. I
would just remove this whole paragraph here, just leave the first one
stating that the tags must be initialised if not already done so.

-- 
Catalin