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

[Luiz Capitulino]

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@xxxxxxxxxx>
> > ---
> >   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.