Re: [RESEND v7 07/29] x86: mm: add PMD swap-exclusive helpers

[Kiryl Shutsemau <kas@kernel.org>]

On Mon, Sep 14, 2026 at 05:27:57AM -0700, Usama Arif wrote:
> A later patch keeps a PMD-mapped anonymous THP mapped by a PMD across the
> swap round-trip, so PG_anon_exclusive now has to survive in a swap PMD and
> not just in a swap PTE.
> 
> x86-64 encodes a swap PMD exactly like a swap PTE, so the new helpers reuse
> _PAGE_SWP_EXCLUSIVE, bit 3, which the swap-entry layout already reserves
> for PG_anon_exclusive. 32-bit x86 aliases that bit to _PAGE_PSE and does
> not select ARCH_HAS_PMD_SOFTLEAVES.
> 
> Cc: Thomas Gleixner <tglx@kernel.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: x86@kernel.org
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>  arch/x86/include/asm/pgtable.h | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index d551120a7c889..a2d1cd03cba23 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1525,6 +1525,23 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
>  	return pte_clear_flags(pte, _PAGE_SWP_EXCLUSIVE);
>  }
>  
> +#ifdef CONFIG_ARCH_HAS_PMD_SOFTLEAVES
> +static inline pmd_t pmd_swp_mkexclusive(pmd_t pmd)
> +{
> +	return pmd_set_flags(pmd, _PAGE_SWP_EXCLUSIVE);
> +}
> +
> +static inline bool pmd_swp_exclusive(pmd_t pmd)
> +{
> +	return pmd_flags(pmd) & _PAGE_SWP_EXCLUSIVE;
> +}
> +
> +static inline pmd_t pmd_swp_clear_exclusive(pmd_t pmd)
> +{
> +	return pmd_clear_flags(pmd, _PAGE_SWP_EXCLUSIVE);
> +}
> +#endif
> +

I was about to comment that it is broken on 32-bit as
_PAGE_SWP_EXCLUSIVE there is _PAGE_PSE which would make exclusive swap
entries pmd_present(). But CONFIG_ARCH_HAS_PMD_SOFTLEAVES is only
selected for X86_64.

Do we want some BUILD_BUG_ON() to catch if anyone would try to enable
it for 32-bit machines?

Otherwise:

Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

>  #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
>  static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
>  {
> -- 
> 2.53.0-Meta
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov