[RFC 05/11] mm/memory/fault: Add support for zone device THP fault handling

[David Hildenbrand]

On 06.03.25 05:42, Balbir Singh wrote:
> When the CPU touches a zone device THP entry, the data needs to
> be migrated back to the CPU, call migrate_to_ram() on these pages
> via do_huge_pmd_device_private() fault handling helper.
> 
> Signed-off-by: Balbir Singh <balbirs at nvidia.com>
> ---
>   include/linux/huge_mm.h |  7 +++++++
>   mm/huge_memory.c        | 35 +++++++++++++++++++++++++++++++++++
>   mm/memory.c             |  6 ++++--
>   3 files changed, 46 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index e893d546a49f..ad0c0ccfcbc2 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -479,6 +479,8 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>   
>   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;
>   
> @@ -634,6 +636,11 @@ 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 d8e018d1bdbd..995ac8be5709 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1375,6 +1375,41 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>   	return __do_huge_pmd_anonymous_page(vmf);
>   }
>   
> +vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf)
> +{
> +	struct vm_area_struct *vma = vmf->vma;
> +	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> +	vm_fault_t ret;
> +	spinlock_t *ptl;
> +	swp_entry_t swp_entry;
> +	struct page *page;
> +
> +	if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
> +		return VM_FAULT_FALLBACK;

I'm confused. Why is that required when we already have a PMD entry?

Apart from that, nothing jumped at me.


-- 
Cheers,

David / dhildenb