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

[Usama Arif <usama.arif@linux.dev>]

On 18/08/2026 15:40, 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.
> 
> (likely the _swp_ part should then be renamed to indicate that this is for
> softdirty entries, not just swap entries)
> 

Agreed. I kept the pmd_swp_* spelling to mirror the existing pte_swp_* API.
Renaming the broader softleaf API is independent of this series. Maybe I can
add it as a followup?

>> 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.
> 

I have added this in my series, and will include it in the next revision, Thanks!

>>
>> Signed-off-by: Usama Arif <usama.arif@linux.dev>
>> ---
>>  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 :)
> 

I have now split the architecture helpers into separate patches, followed by generic
support for the next revision. I think I will need split patch 7 and patch 12 into
several independent patches as well? This will significantly grow the patch count
which is what I was trying to avoid as I feel that makes it more daunting to review
(atleast for me), but I will try and keep it under 57 patches to not beat Kiryl :)

> [...]
> 
>>  #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.
> 

Yes, we can, will do in next revision.

> 
>>  	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 :)
> 
Will do in next revision.

Thanks for the reviews!

>>  	return softleaf_is_device_private(entry) ||
>> -		softleaf_is_migration(entry);
>> +		softleaf_is_migration(entry) ||
>> +		softleaf_is_swap(entry);
>> +}
>> +
>