Re: [PATCH v6 02/12] mm: add PMD swap entry detection support

[Lorenzo Stoakes (ARM)]

On Tue, Aug 18, 2026 at 04:40:25PM +0200, David Hildenbrand (Arm) wrote:
> On 8/18/26 15:09, Usama Arif wrote:
> > Currently when a PMD-mapped THP is swapped out, the PMD is always
> > split into HPAGE_PMD_NR PTE-level swap entries. To preserve huge
> > page information across swap cycles, later patches will install a
> > single PMD-level swap entry instead. Add the infrastructure to detect
> > those entries.
> >
> > Teach the softleaf layer to recognise PMD swap entries:
> > pmd_is_swap_entry() detects them and softleaf_is_valid_pmd_entry()
> > accepts them as a valid non-present type. Because swap entries do not
> > encode a PFN, make pmd_softleaf_to_folio() warn and return NULL for them
> > instead of passing the swap offset to softleaf_to_folio(). Clear the
> > exclusive overlay bit in softleaf_from_pmd() before decoding, matching
> > how soft_dirty and uffd bits are already stripped.
> >
> > Add pmd_swp_mkexclusive(), pmd_swp_exclusive(), and
> > pmd_swp_clear_exclusive() helpers to each architecture that supports
> > PMD softleaf entries (x86, arm64, s390, riscv, loongarch, powerpc),
> > mirroring the existing PTE swap exclusive helpers in each arch's
> > pgtable.h.
>
> Ah, for migration entries we still use a dedicated migratetype. I actually have
> on my todo list to move to PTE bits as well.

We do seem to have rather hacked in PMD level stuff across the board
previously :)

>
> (likely the _swp_ part should then be renamed to indicate that this is for
> softdirty entries, not just swap entries)

You mean softleaf?

>
> > Provide generic no-op PMD swap exclusive fallbacks for
> > architectures without PMD softleaf support, matching the generic PMD
> > swap soft-dirty fallbacks.
>
> No softleaf implies to migration and no swap, so this would work.
>
> You should extend mm/debug_vm_pgtable.c in a separate commit to test what
> pte_swap_exclusive_tests() tests for PMDs.

BTW one thing I should maybe mention here is that I never quite _finished_
the softleaf thing, so there's some awkward overlap between swap stuff and
softleaf stuff (swap entries _are_ softleaf entries, just a specific kind).

Infinite TODO list etc. (maybe this is a hint/nudge to somebody else to do
it ;)

>
> >
> > Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/pgtable.h             |  6 +++++
> >  arch/loongarch/include/asm/pgtable.h         | 19 ++++++++++++++
> >  arch/powerpc/include/asm/book3s/64/pgtable.h | 17 +++++++++++++
> >  arch/riscv/include/asm/pgtable.h             | 15 +++++++++++
> >  arch/s390/include/asm/pgtable.h              | 17 +++++++++++++
> >  arch/x86/include/asm/pgtable.h               | 17 +++++++++++++
>
> I'm sorry for asking you to compete with Kiryll's series by creating one patch
> for each architecture that directly jumps at arch maintainers :)
>
> [...]
>
> >  #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
> >  static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
> >  {
> > diff --git a/include/linux/leafops.h b/include/linux/leafops.h
> > index 7c13c58a5e218..4a6c52974b305 100644
> > --- a/include/linux/leafops.h
> > +++ b/include/linux/leafops.h
> > @@ -102,6 +102,8 @@ static inline softleaf_t softleaf_from_pmd(pmd_t pmd)
> >  		pmd = pmd_swp_clear_soft_dirty(pmd);
> >  	if (pmd_swp_uffd(pmd))
> >  		pmd = pmd_swp_clear_uffd(pmd);
> > +	if (pmd_swp_exclusive(pmd))
> > +		pmd = pmd_swp_clear_exclusive(pmd);
>
> Can't we just unconditionally clear these flags?
>
> pmd = pmd_swp_clear_uffd(pmd);
> pmd = pmd_swp_clear_exclusive(pmd);
>
> Avoids these rather unnecessary conditionals unless I am missing something.
>
>
> >  	arch_entry = __pmd_to_swp_entry(pmd);
> >
> >  	/* Temporary until swp_entry_t eliminated. */
> > @@ -634,18 +636,30 @@ static inline bool pmd_is_migration_entry(pmd_t pmd)
> >   */
> >  static inline bool softleaf_is_valid_pmd_entry(softleaf_t entry)
> >  {
> > -	/* Only device private, migration entries valid for PMD. */
> > +	/* Device private, migration, and swap entries valid for PMD. */
>
> Can we just drop that comment? I mean, it's as clear as it gets in the code
> immediately below :)
>
> >  	return softleaf_is_device_private(entry) ||
> > -		softleaf_is_migration(entry);
> > +		softleaf_is_migration(entry) ||
> > +		softleaf_is_swap(entry);
> > +}
> > +
>
> --
> Cheers,
>
> David

--
Cheers, Lorenzo