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

[Usama Arif]

On 18/08/2026 19:42, Lorenzo Stoakes (ARM) wrote:
> 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?
> 

Yes, I read that as softleaf as well.>>
>>> 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 ;)
> 

I kind of tried to tackle some this along the way, with the prep series in
[1]

[1] https://lore.kernel.org/all/20260706114320.1643046-1-usama.arif@xxxxxxxxx/#t


>>
>>>
>>> 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