Re: [v5 07/15] mm/memory/fault: add THP fault handling for zone device private pages

[Balbir Singh]

On 9/11/25 22:42, David Hildenbrand wrote:
> On 08.09.25 02:04, Balbir Singh wrote:
>> Implement CPU fault handling for zone device THP entries through
>> do_huge_pmd_device_private(), enabling transparent migration of
>> device-private large pages back to system memory on CPU access.
>>
>> When the CPU accesses a zone device THP entry, the fault handler
>> calls the device driver's migrate_to_ram() callback to migrate
>> the entire large page back to system memory.
>>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>> Cc: Zi Yan <ziy@xxxxxxxxxx>
>> Cc: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
>> Cc: Rakie Kim <rakie.kim@xxxxxx>
>> Cc: Byungchul Park <byungchul@xxxxxx>
>> Cc: Gregory Price <gourry@xxxxxxxxxx>
>> Cc: Ying Huang <ying.huang@xxxxxxxxxxxxxxxxx>
>> Cc: Alistair Popple <apopple@xxxxxxxxxx>
>> Cc: Oscar Salvador <osalvador@xxxxxxx>
>> Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
>> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>> Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
>> Cc: Nico Pache <npache@xxxxxxxxxx>
>> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
>> Cc: Dev Jain <dev.jain@xxxxxxx>
>> Cc: Barry Song <baohua@xxxxxxxxxx>
>> Cc: Lyude Paul <lyude@xxxxxxxxxx>
>> Cc: Danilo Krummrich <dakr@xxxxxxxxxx>
>> Cc: David Airlie <airlied@xxxxxxxxx>
>> Cc: Simona Vetter <simona@xxxxxxxx>
>> Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
>> Cc: Mika Penttilä <mpenttil@xxxxxxxxxx>
>> Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
>> Cc: Francois Dugast <francois.dugast@xxxxxxxxx>
>>
>> Signed-off-by: Balbir Singh <balbirs@xxxxxxxxxx>
>> ---
>>   include/linux/huge_mm.h |  7 +++++++
>>   mm/huge_memory.c        | 36 ++++++++++++++++++++++++++++++++++++
>>   mm/memory.c             |  6 ++++--
>>   3 files changed, 47 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index 23f124493c47..2c6a0c3c862c 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -496,6 +496,8 @@ static inline bool folio_test_pmd_mappable(struct folio *folio)
>>     vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf);
>>   +vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf);
>> +
>>   extern struct folio *huge_zero_folio;
>>   extern unsigned long huge_zero_pfn;
>>   @@ -675,6 +677,11 @@ static inline vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>>       return 0;
>>   }
>>   +static inline vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf)
>> +{
> 
> Is this a VM_WARN_ON_ONCE() or similar? (Maybe BUILD_BUG is possible?)
> 
>> +    return 0;
>> +}
>> +
>>   static inline bool is_huge_zero_folio(const struct folio *folio)
>>   {
>>       return false;
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index b720870c04b2..d634b2157a56 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1287,6 +1287,42 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>>     }
>>   +vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf)
>> +{
>> +    struct vm_area_struct *vma = vmf->vma;
>> +    vm_fault_t ret = 0;
>> +    spinlock_t *ptl;
>> +    swp_entry_t swp_entry;
>> +    struct page *page;
>> +
>> +    if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
>> +        vma_end_read(vma);
>> +        return VM_FAULT_RETRY;
>> +    }
>> +
>> +    ptl = pmd_lock(vma->vm_mm, vmf->pmd);
>> +    if (unlikely(!pmd_same(*vmf->pmd, vmf->orig_pmd))) {
>> +        spin_unlock(ptl);
>> +        return 0;
>> +    }
>> +
>> +    swp_entry = pmd_to_swp_entry(vmf->orig_pmd);
>> +    page = pfn_swap_entry_to_page(swp_entry);
>> +    vmf->page = page;
>> +    vmf->pte = NULL;
>> +    if (trylock_page(vmf->page)) {
>> +        get_page(page);
>> +        spin_unlock(ptl);
>> +        ret = page_pgmap(page)->ops->migrate_to_ram(vmf);
>> +        unlock_page(vmf->page);
>> +        put_page(page);
>> +    } else {
>> +        spin_unlock(ptl);
>> +    }
>> +
>> +    return ret;
>> +}
>> +
>>   /*
>>    * always: directly stall for all thp allocations
>>    * defer: wake kswapd and fail if not immediately available
>> diff --git a/mm/memory.c b/mm/memory.c
>> index d9de6c056179..860665f4b692 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -6298,8 +6298,10 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>>           vmf.orig_pmd = pmdp_get_lockless(vmf.pmd);
>>             if (unlikely(is_swap_pmd(vmf.orig_pmd))) {
>> -            VM_BUG_ON(thp_migration_supported() &&
>> -                      !is_pmd_migration_entry(vmf.orig_pmd));
>> +            if (is_device_private_entry(
>> +                    pmd_to_swp_entry(vmf.orig_pmd)))
> 
> Single line please.

Ack

> 
> But didn't we have a pmd helper for that?
> 

This is a single if that handles is_swap_pmd() and then is_device_private_entry() and is_pmd_migration_entry()
under that

>> +                return do_huge_pmd_device_private(&vmf);
>> +
>>               if (is_pmd_migration_entry(vmf.orig_pmd))
>>                   pmd_migration_entry_wait(mm, vmf.pmd);
>>               return 0;
> 
> 

Thanks,
Balbir