Re: [v6 07/15] mm/memory/fault: add THP fault handling for zone device private pages

[David Hildenbrand <david@redhat.com>]

On 16.09.25 14:21, 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.
> 
> 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/huge_mm.h |  7 +++++++
>   mm/huge_memory.c        | 36 ++++++++++++++++++++++++++++++++++++
>   mm/memory.c             |  5 +++--
>   3 files changed, 46 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index f327d62fc985..2d669be7f1c8 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;
>   
> @@ -671,6 +673,11 @@ 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)
> +{
> +	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 5291ee155a02..90a1939455dd 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)) {

We should be operating on a folio here. folio_trylock() + folio_get() + 
folio_unlock() + folio_put().

> +		get_page(page);
> +		spin_unlock(ptl);
> +		ret = page_pgmap(page)->ops->migrate_to_ram(vmf);

BTW, I was wondering whether it is really the right design to pass the 
vmf here. Likely the const vma+addr+folio could be sufficient. I did not 
look into all callbaks, though.

-- 
Cheers

David / dhildenb