linux-kernel - Re: [PATCH] arm64/mm: Drop redundant addr increment in set_huge_pte

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Message-ID: <e10e0fac-a81c-445e-8036-9db2b63b77f7@arm.com>
Date: Tue, 15 Jul 2025 11:28:28 +0530
From: Dev Jain <dev.jain@....com>
To: Anshuman Khandual <anshuman.khandual@....com>,
 linux-arm-kernel@...ts.infradead.org
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Ryan Roberts <ryan.roberts@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: Drop redundant addr increment in
 set_huge_pte_at()


On 15/07/25 10:09 am, Anshuman Khandual wrote:
> The 'addr' need not be incremented while operating on single entry basis as
> BBM is not required for such updates.

"while operating on a single entry basis" should be replaced with
"when transitioning from invalid to valid pte"; BBM is required
for single entry updates when the cont bit is set, as described
in the comment immediately below. In fact, let us not even mention
BBM at all - the reasoning is that the code is written in a way
that, the loop incrementing addr, immediately has a return after
it, and it does not actually use the addr, so let's drop the
increment. It is not immediately obvious from the commit description
that the addr is passed ultimately to flush_tlb_range friends which
is involved in BBM.

Reviewed-by: Dev Jain <dev.jain@....com>

>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
>   arch/arm64/mm/hugetlbpage.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index 0c8737f4f2ce..1d90a7e75333 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -225,7 +225,7 @@ void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
>   	ncontig = num_contig_ptes(sz, &pgsize);
>   
>   	if (!pte_present(pte)) {
> -		for (i = 0; i < ncontig; i++, ptep++, addr += pgsize)
> +		for (i = 0; i < ncontig; i++, ptep++)
>   			__set_ptes_anysz(mm, ptep, pte, 1, pgsize);
>   		return;
>   	}