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

[David Hildenbrand <david@redhat.com>]

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@linux-foundation.org>
> 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>
> 
> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
> ---
>   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.

But didn't we have a pmd helper for 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;


-- 
Cheers

David / dhildenb