Re: [v6 02/15] mm/huge_memory: add device-private THP support to PMD operations

[David Hildenbrand <david@redhat.com>]

On 25.09.25 02:25, Alistair Popple wrote:
> On 2025-09-16 at 22:21 +1000, Balbir Singh <balbirs@nvidia.com> wrote...
>> Extend core huge page management functions to handle device-private THP
>> entries.  This enables proper handling of large device-private folios in
>> fundamental MM operations.
>>
>> The following functions have been updated:
>>
>> - copy_huge_pmd(): Handle device-private entries during fork/clone
>> - zap_huge_pmd(): Properly free device-private THP during munmap
>> - change_huge_pmd(): Support protection changes on device-private THP
>> - __pte_offset_map(): Add device-private entry awareness
>>
>> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
>> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
>> Cc: Rakie Kim <rakie.kim@sk.com>
>> Cc: Byungchul Park <byungchul@sk.com>
>> Cc: Gregory Price <gourry@gourry.net>
>> Cc: Ying Huang <ying.huang@linux.alibaba.com>
>> Cc: Alistair Popple <apopple@nvidia.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
>> Cc: Nico Pache <npache@redhat.com>
>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>> Cc: Dev Jain <dev.jain@arm.com>
>> Cc: Barry Song <baohua@kernel.org>
>> Cc: Lyude Paul <lyude@redhat.com>
>> Cc: Danilo Krummrich <dakr@kernel.org>
>> Cc: David Airlie <airlied@gmail.com>
>> Cc: Simona Vetter <simona@ffwll.ch>
>> Cc: Ralph Campbell <rcampbell@nvidia.com>
>> Cc: Mika Penttilä <mpenttil@redhat.com>
>> Cc: Matthew Brost <matthew.brost@intel.com>
>> Cc: Francois Dugast <francois.dugast@intel.com>
>> ---
>>   include/linux/swapops.h | 32 +++++++++++++++++++++++
>>   mm/huge_memory.c        | 56 ++++++++++++++++++++++++++++++++++-------
>>   mm/pgtable-generic.c    |  2 +-
>>   3 files changed, 80 insertions(+), 10 deletions(-)
>>
>> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
>> index 64ea151a7ae3..2687928a8146 100644
>> --- a/include/linux/swapops.h
>> +++ b/include/linux/swapops.h
>> @@ -594,10 +594,42 @@ static inline int is_pmd_migration_entry(pmd_t pmd)
>>   }
>>   #endif  /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
>>   
>> +#if defined(CONFIG_ZONE_DEVICE) && defined(CONFIG_ARCH_ENABLE_THP_MIGRATION)
>> +
>> +/**
>> + * is_pmd_device_private_entry() - Check if PMD contains a device private swap entry
>> + * @pmd: The PMD to check
>> + *
>> + * Returns true if the PMD contains a swap entry that represents a device private
>> + * page mapping. This is used for zone device private pages that have been
>> + * swapped out but still need special handling during various memory management
>> + * operations.
>> + *
>> + * Return: 1 if PMD contains device private entry, 0 otherwise
>> + */
>> +static inline int is_pmd_device_private_entry(pmd_t pmd)
>> +{
>> +	return is_swap_pmd(pmd) && is_device_private_entry(pmd_to_swp_entry(pmd));
>> +}
>> +
>> +#else /* CONFIG_ZONE_DEVICE && CONFIG_ARCH_ENABLE_THP_MIGRATION */
>> +
>> +static inline int is_pmd_device_private_entry(pmd_t pmd)
>> +{
>> +	return 0;
>> +}
>> +
>> +#endif /* CONFIG_ZONE_DEVICE && CONFIG_ARCH_ENABLE_THP_MIGRATION */
>> +
>>   static inline int non_swap_entry(swp_entry_t entry)
>>   {
>>   	return swp_type(entry) >= MAX_SWAPFILES;
>>   }
>>   
>> +static inline int is_pmd_non_present_folio_entry(pmd_t pmd)
> 
> I can't think of a better name either although I am curious why open-coding it
> was so nasty given we don't have the equivalent for pte entries. Will go read
> the previous discussion.

I think for PTEs we just handle all cases (markers, hwpoison etc) 
properly, manye not being supported yet on the PMD level. See 
copy_nonpresent_pte() as an example.

We don't even have helpers like is_pte_migration_entry().

>> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
>> index 567e2d084071..0c847cdf4fd3 100644
>> --- a/mm/pgtable-generic.c
>> +++ b/mm/pgtable-generic.c
>> @@ -290,7 +290,7 @@ pte_t *___pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
>>   
>>   	if (pmdvalp)
>>   		*pmdvalp = pmdval;
>> -	if (unlikely(pmd_none(pmdval) || is_pmd_migration_entry(pmdval)))
>> +	if (unlikely(pmd_none(pmdval) || !pmd_present(pmdval)))
> 
> Why isn't is_pmd_non_present_folio_entry() used here?


I thought I argued that

	if (!pmd_present(pmdval)))

Should be sufficient here in my last review?

We want to detect page tables we can map after all.
-- 
Cheers

David / dhildenb