Re: [PATCH v8 07/30] x86: mm: add PMD swap-exclusive helpers

[Usama Arif]

On Mon, 05 Oct 2026 16:14:52 -0400 "Zi Yan" <ziy@xxxxxxxxxx> wrote:

> On Fri Oct 2, 2026 at 5:52 AM EDT, Usama Arif wrote:
> > Prepare for keeping a PMD-mapped anonymous THP mapped by a PMD across the
> > swap round-trip, which makes PG_anon_exclusive have 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; assert that, because an exclusive swap
> > PMD would otherwise be pmd_present().
> >
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
> > Reviewed-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> > Reviewed-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/pgtable.h | 20 ++++++++++++++++++++
> >  1 file changed, 20 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> > index d551120a7c889..c35dfa6ceda05 100644
> > --- a/arch/x86/include/asm/pgtable.h
> > +++ b/arch/x86/include/asm/pgtable.h
> > @@ -1525,6 +1525,26 @@ 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
> > +/* 32-bit aliases this to _PAGE_PSE, making an exclusive swap PMD present. */
> 
> Is it better to say? Since the above comment sounds like 32-bit
> specific, but the actual constraint is about not being _PAGE_PSE.
> 
> _PAGE_SWP_EXCLUSIVE cannot be _PAGE_PSE, otherwise making an exclusive
> swap PMD present.

Updated the comment to:

/*
 * _PAGE_SWP_EXCLUSIVE must not alias _PAGE_PSE, which would make an
 * exclusive swap PMD present.
 */

which I think is clearer.

> 
> > +static_assert(_PAGE_SWP_EXCLUSIVE != _PAGE_PSE);
> > +
> > +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
> > +
> >  #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
> >  static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
> >  {
> 
> Otherwise, LGTM.
> 
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

Thanks for the review! And apologies for the late reply, I was away.

> 
> 
> -- 
> Best Regards,
> Yan, Zi
> 
>