linux-kernel - Re: [PATCH] arm64: mm: Drop duplicate check in pmd_trans

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Message-ID: <4e5941f8-7e61-4a63-a669-bee1601093a6@arm.com>
Date: Thu, 8 May 2025 11:14:20 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Gavin Shan <gshan@...hat.com>, linux-arm-kernel@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org, catalin.marinas@....com, will@...nel.org,
 ryan.roberts@....com, peterx@...hat.com, joey.gouly@....com,
 yangyicong@...ilicon.com, shan.gavin@...il.com
Subject: Re: [PATCH] arm64: mm: Drop duplicate check in pmd_trans_huge()

On 5/8/25 09:21, Gavin Shan wrote:
> pmd_val(pmd) is inclusive to pmd_present(pmd) since the PMD entry
> value isn't zero when pmd_present() returns true. Just drop the
> duplicate check done by pmd_val(pmd).

Agreed, pmd_val() is redundant here because a positive pmd_present()
also ensures a positive pmd_val().

#define pmd_present(pmd) pte_present(pmd_pte(pmd))
#define pte_present(pte) (pte_valid(pte) || pte_present_invalid(pte))

#define pte_valid(pte)           (!!(pte_val(pte) & PTE_VALID))
#define pte_present_invalid(pte) ((pte_val(pte) & (PTE_VALID |
				  PTE_PRESENT_INVALID)) == PTE_PRESENT_INVALID)

pte_present() cannot return positive here unless either of the flags
PTE_VALID or PTE_PRESENT_INVALID is set which implies pte_val() would
also return positive.

Probably it would be better to add the above details in the commit
message here as well.

The earlier commit skipped dropping pmd_val() in order to keep then
proposed change confined to just adding new pmd_table() check, even
though pmd_val() redundancy was evident as well which should have
been dropped there after.

d1770e909898 ("arm64/mm: Check pmd_table() in pmd_trans_huge()")

> 
> No functional changes intended.
> 
> Signed-off-by: Gavin Shan <gshan@...hat.com>
> ---
> Found this by code inspection
> ---
>  arch/arm64/include/asm/pgtable.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index d3b538be1500..2599b9b8666f 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -739,8 +739,7 @@ static inline int pmd_trans_huge(pmd_t pmd)
>  	 * If pmd is present-invalid, pmd_table() won't detect it
>  	 * as a table, so force the valid bit for the comparison.
>  	 */
> -	return pmd_val(pmd) && pmd_present(pmd) &&
> -	       !pmd_table(__pmd(pmd_val(pmd) | PTE_VALID));
> +	return pmd_present(pmd) && !pmd_table(__pmd(pmd_val(pmd) | PTE_VALID));
>  }
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> 
Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>