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

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Message-ID: <fdeff5e5-daab-4967-a6bf-f71efd78507c@redhat.com>
Date: Thu, 8 May 2025 18:56:17 +1000
From: Gavin Shan <gshan@...hat.com>
To: Anshuman Khandual <anshuman.khandual@....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 3:44 PM, Anshuman Khandual wrote:
> 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.
> 

Thanks, I've squashed those details to v2, which was just posted.

> 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()")
> 

Yes, agreed.

>>
>> 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>
> 

Thanks,
Gavin