linux-kernel - Re: [RFC 10/10] mm: thp: x86: cleanup PSE feature bit usage

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Message-ID: <fe0ced4d-170b-4144-84f9-88fcb4722936@redhat.com>
Date: Mon, 17 Nov 2025 14:15:41 -0500
From: Luiz Capitulino <luizcap@...hat.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc: ryan.roberts@....com, akpm@...ux-foundation.org,
 lorenzo.stoakes@...cle.com
Subject: Re: [RFC 10/10] mm: thp: x86: cleanup PSE feature bit usage

On 2025-11-17 12:49, David Hildenbrand (Red Hat) wrote:
> On 06.11.25 22:28, Luiz Capitulino wrote:
>> Historically, THP support on x86 checked the PSE feature bit to enable
>> THP. On 64-bit, this check is redundant since PSE is always enabled by
>> default for compatibility. On 32-bit, PSE enables 4MB page support and
> 
> Or 2MB support with 4byte PTEs I guess?
> 
>> must be checked. To clean this up, this commit:
>>
> 
> Likely the subject should start with: "x86/mm:"

You're right on both points. I'll fix the changelog for the next
version.

> 
> 
>> 1. Drops arch_has_pmd_leaves() from common x86 code. For 64-bit,
>>     we assume PMD-sized pages are always supported
>>
>> 2. Checks for PSE only on 32-bit, using arch_has_pmd_leaves()
>>
>> Signed-off-by: Luiz Capitulino <luizcap@...hat.com>
>> ---
>>   arch/x86/include/asm/pgtable.h    | 6 ------
>>   arch/x86/include/asm/pgtable_32.h | 6 ++++++
>>   2 files changed, 6 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
>> index 08d109280e36..55b88de5178f 100644
>> --- a/arch/x86/include/asm/pgtable.h
>> +++ b/arch/x86/include/asm/pgtable.h
>> @@ -313,12 +313,6 @@ static inline int pud_trans_huge(pud_t pud)
>>   }
>>   #endif
>> -#define arch_has_pmd_leaves arch_has_pmd_leaves
>> -static inline int arch_has_pmd_leaves(void)
>> -{
>> -    return boot_cpu_has(X86_FEATURE_PSE);
>> -}
>> -
>>   #ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
>>   static inline bool pmd_special(pmd_t pmd)
>>   {
>> diff --git a/arch/x86/include/asm/pgtable_32.h b/arch/x86/include/asm/pgtable_32.h
>> index b612cc57a4d3..3bd51cfa431e 100644
>> --- a/arch/x86/include/asm/pgtable_32.h
>> +++ b/arch/x86/include/asm/pgtable_32.h
>> @@ -45,6 +45,12 @@ do {                        \
>>       flush_tlb_one_kernel((vaddr));        \
>>   } while (0)
>> +#define arch_has_pmd_leaves arch_has_pmd_leaves
>> +static inline int arch_has_pmd_leaves(void)
>> +{
>> +    return boot_cpu_has(X86_FEATURE_PSE);
>> +}
>> +
>>   #endif /* !__ASSEMBLER__ */
>>   /*
> 
> Right, IIUC 64bit should neve rrequire this check.
>