Message-ID: <FB8D8651-639F-4882-9868-7FA0A8B62397@nvidia.com>
Date: Mon, 13 Oct 2025 17:17:44 -0400
From: Zi Yan <ziy@...dia.com>
To: Balbir Singh <balbirs@...dia.com>
Cc: linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linux-mm@...ck.org, akpm@...ux-foundation.org,
 David Hildenbrand <david@...hat.com>, Joshua Hahn <joshua.hahnjy@...il.com>,
 Rakie Kim <rakie.kim@...com>, Byungchul Park <byungchul@...com>,
 Gregory Price <gourry@...rry.net>, Ying Huang <ying.huang@...ux.alibaba.com>,
 Alistair Popple <apopple@...dia.com>, Oscar Salvador <osalvador@...e.de>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Ralph Campbell <rcampbell@...dia.com>,
 Mika Penttilä <mpenttil@...hat.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v7 11/16] mm/migrate_device: add THP splitting during migration

On 1 Oct 2025, at 2:57, Balbir Singh wrote:

> Implement migrate_vma_split_pages() to handle THP splitting during the
> migration process when destination cannot allocate compound pages.
>
> This addresses the common scenario where migrate_vma_setup() succeeds with
> MIGRATE_PFN_COMPOUND pages, but the destination device cannot allocate
> large pages during the migration phase.
>
> Key changes:
> - migrate_vma_split_pages(): Split already-isolated pages during migration
> - Enhanced folio_split() and __split_unmapped_folio() with isolated
>   parameter to avoid redundant unmap/remap operations
>
> This provides a fallback mechansim to ensure migration succeeds even when
> large page allocation fails at the destination.
>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Joshua Hahn <joshua.hahnjy@...il.com>
> Cc: Rakie Kim <rakie.kim@...com>
> Cc: Byungchul Park <byungchul@...com>
> Cc: Gregory Price <gourry@...rry.net>
> Cc: Ying Huang <ying.huang@...ux.alibaba.com>
> Cc: Alistair Popple <apopple@...dia.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Nico Pache <npache@...hat.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Dev Jain <dev.jain@....com>
> Cc: Barry Song <baohua@...nel.org>
> Cc: Lyude Paul <lyude@...hat.com>
> Cc: Danilo Krummrich <dakr@...nel.org>
> Cc: David Airlie <airlied@...il.com>
> Cc: Simona Vetter <simona@...ll.ch>
> Cc: Ralph Campbell <rcampbell@...dia.com>
> Cc: Mika Penttilä <mpenttil@...hat.com>
> Cc: Matthew Brost <matthew.brost@...el.com>
> Cc: Francois Dugast <francois.dugast@...el.com>
>
> Signed-off-by: Balbir Singh <balbirs@...dia.com>
> ---
>  include/linux/huge_mm.h | 11 +++++-
>  lib/test_hmm.c          |  9 +++++
>  mm/huge_memory.c        | 46 ++++++++++++----------
>  mm/migrate_device.c     | 85 +++++++++++++++++++++++++++++++++++------
>  4 files changed, 117 insertions(+), 34 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 2d669be7f1c8..a166be872628 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -365,8 +365,8 @@ unsigned long thp_get_unmapped_area_vmflags(struct file *filp, unsigned long add
>  		vm_flags_t vm_flags);
>
>  bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
> -int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> -		unsigned int new_order);
> +int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> +		unsigned int new_order, bool unmapped);
>  int min_order_for_split(struct folio *folio);
>  int split_folio_to_list(struct folio *folio, struct list_head *list);
>  bool uniform_split_supported(struct folio *folio, unsigned int new_order,
> @@ -375,6 +375,13 @@ bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
>  		bool warns);
>  int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
>  		struct list_head *list);
> +
> +static inline int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> +		unsigned int new_order)
> +{
> +	return __split_huge_page_to_list_to_order(page, list, new_order, false);
> +}
> +
>  /*
>   * try_folio_split - try to split a @folio at @page using non uniform split.
>   * @folio: folio to be split
> diff --git a/lib/test_hmm.c b/lib/test_hmm.c
> index 46fa9e200db8..df429670633e 100644
> --- a/lib/test_hmm.c
> +++ b/lib/test_hmm.c
> @@ -1612,6 +1612,15 @@ static vm_fault_t dmirror_devmem_fault(struct vm_fault *vmf)
>  	order = folio_order(page_folio(vmf->page));
>  	nr = 1 << order;
>
> +	/*
> +	 * When folios are partially mapped, we can't rely on the folio
> +	 * order of vmf->page as the folio might not be fully split yet
> +	 */
> +	if (vmf->pte) {
> +		order = 0;
> +		nr = 1;
> +	}
> +
>  	/*
>  	 * Consider a per-cpu cache of src and dst pfns, but with
>  	 * large number of cpus that might not scale well.
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 8c95a658b3ec..022b0729f826 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3463,15 +3463,6 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>  		new_folio->mapping = folio->mapping;
>  		new_folio->index = folio->index + i;
>
> -		/*
> -		 * page->private should not be set in tail pages. Fix up and warn once
> -		 * if private is unexpectedly set.
> -		 */
> -		if (unlikely(new_folio->private)) {
> -			VM_WARN_ON_ONCE_PAGE(true, new_head);
> -			new_folio->private = NULL;
> -		}
> -
>  		if (folio_test_swapcache(folio))
>  			new_folio->swap.val = folio->swap.val + i;
>
> @@ -3700,6 +3691,7 @@ bool uniform_split_supported(struct folio *folio, unsigned int new_order,
>   * @lock_at: a page within @folio to be left locked to caller
>   * @list: after-split folios will be put on it if non NULL
>   * @uniform_split: perform uniform split or not (non-uniform split)
> + * @unmapped: The pages are already unmapped, they are migration entries.
>   *
>   * It calls __split_unmapped_folio() to perform uniform and non-uniform split.
>   * It is in charge of checking whether the split is supported or not and
> @@ -3715,7 +3707,7 @@ bool uniform_split_supported(struct folio *folio, unsigned int new_order,
>   */
>  static int __folio_split(struct folio *folio, unsigned int new_order,
>  		struct page *split_at, struct page *lock_at,
> -		struct list_head *list, bool uniform_split)
> +		struct list_head *list, bool uniform_split, bool unmapped)
>  {
>  	struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>  	XA_STATE(xas, &folio->mapping->i_pages, folio->index);
> @@ -3765,13 +3757,15 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  		 * is taken to serialise against parallel split or collapse
>  		 * operations.
>  		 */
> -		anon_vma = folio_get_anon_vma(folio);
> -		if (!anon_vma) {
> -			ret = -EBUSY;
> -			goto out;
> +		if (!unmapped) {
> +			anon_vma = folio_get_anon_vma(folio);
> +			if (!anon_vma) {
> +				ret = -EBUSY;
> +				goto out;
> +			}
> +			anon_vma_lock_write(anon_vma);
>  		}
>  		mapping = NULL;
> -		anon_vma_lock_write(anon_vma);
>  	} else {
>  		unsigned int min_order;
>  		gfp_t gfp;
> @@ -3838,7 +3832,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  		goto out_unlock;
>  	}
>
> -	unmap_folio(folio);
> +	if (!unmapped)
> +		unmap_folio(folio);
>
>  	/* block interrupt reentry in xa_lock and spinlock */
>  	local_irq_disable();
> @@ -3925,10 +3920,13 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>
>  			next = folio_next(new_folio);
>
> +			zone_device_private_split_cb(folio, new_folio);
> +
>  			expected_refs = folio_expected_ref_count(new_folio) + 1;
>  			folio_ref_unfreeze(new_folio, expected_refs);
>
> -			lru_add_split_folio(folio, new_folio, lruvec, list);
> +			if (!unmapped)
> +				lru_add_split_folio(folio, new_folio, lruvec, list);
>
>  			/*
>  			 * Anonymous folio with swap cache.
> @@ -3959,6 +3957,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  			__filemap_remove_folio(new_folio, NULL);
>  			folio_put_refs(new_folio, nr_pages);
>  		}
> +
> +		zone_device_private_split_cb(folio, NULL);
>  		/*
>  		 * Unfreeze @folio only after all page cache entries, which
>  		 * used to point to it, have been updated with new folios.
> @@ -3982,6 +3982,9 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>
>  	local_irq_enable();
>
> +	if (unmapped)
> +		return ret;
> +
>  	if (nr_shmem_dropped)
>  		shmem_uncharge(mapping->host, nr_shmem_dropped);
>
> @@ -4072,12 +4075,13 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>   * Returns -EINVAL when trying to split to an order that is incompatible
>   * with the folio. Splitting to order 0 is compatible with all folios.
>   */
> -int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> -				     unsigned int new_order)
> +int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> +				     unsigned int new_order, bool unmapped)
>  {
>  	struct folio *folio = page_folio(page);
>
> -	return __folio_split(folio, new_order, &folio->page, page, list, true);
> +	return __folio_split(folio, new_order, &folio->page, page, list, true,
> +				unmapped);
>  }
>
>  /*
> @@ -4106,7 +4110,7 @@ int folio_split(struct folio *folio, unsigned int new_order,
>  		struct page *split_at, struct list_head *list)
>  {
>  	return __folio_split(folio, new_order, split_at, &folio->page, list,
> -			false);
> +			false, false);
>  }
>
>  int min_order_for_split(struct folio *folio)
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 4156fd6190d2..fa42d2ebd024 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -306,6 +306,23 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			    pgmap->owner != migrate->pgmap_owner)
>  				goto next;
>
> +			folio = page_folio(page);
> +			if (folio_test_large(folio)) {
> +				int ret;
> +
> +				pte_unmap_unlock(ptep, ptl);
> +				ret = migrate_vma_split_folio(folio,
> +							  migrate->fault_page);
> +
> +				if (ret) {
> +					ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> +					goto next;
> +				}
> +
> +				addr = start;
> +				goto again;
> +			}
> +
>  			mpfn = migrate_pfn(page_to_pfn(page)) |
>  					MIGRATE_PFN_MIGRATE;
>  			if (is_writable_device_private_entry(entry))
> @@ -880,6 +897,29 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
>  		src[i] &= ~MIGRATE_PFN_MIGRATE;
>  	return 0;
>  }
> +
> +static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
> +					    unsigned long idx, unsigned long addr,
> +					    struct folio *folio)
> +{
> +	unsigned long i;
> +	unsigned long pfn;
> +	unsigned long flags;
> +	int ret = 0;
> +
> +	folio_get(folio);
> +	split_huge_pmd_address(migrate->vma, addr, true);
> +	ret = __split_huge_page_to_list_to_order(folio_page(folio, 0), NULL,
> +							0, true);

Why not just call __split_unmapped_folio() here? Then, you do not need to add
a new unmapped parameter in __folio_split().


> +	if (ret)
> +		return ret;
> +	migrate->src[idx] &= ~MIGRATE_PFN_COMPOUND;
> +	flags = migrate->src[idx] & ((1UL << MIGRATE_PFN_SHIFT) - 1);
> +	pfn = migrate->src[idx] >> MIGRATE_PFN_SHIFT;
> +	for (i = 1; i < HPAGE_PMD_NR; i++)
> +		migrate->src[i+idx] = migrate_pfn(pfn + i) | flags;
> +	return ret;
> +}
>  #else /* !CONFIG_ARCH_ENABLE_THP_MIGRATION */
>  static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
>  					 unsigned long addr,
> @@ -889,6 +929,13 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
>  {
>  	return 0;
>  }
> +
> +static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
> +					    unsigned long idx, unsigned long addr,
> +					    struct folio *folio)
> +{
> +	return 0;
> +}
>  #endif
>
>  static unsigned long migrate_vma_nr_pages(unsigned long *src)
> @@ -1050,8 +1097,9 @@ static void __migrate_device_pages(unsigned long *src_pfns,
>  				struct migrate_vma *migrate)
>  {
>  	struct mmu_notifier_range range;
> -	unsigned long i;
> +	unsigned long i, j;
>  	bool notified = false;
> +	unsigned long addr;
>
>  	for (i = 0; i < npages; ) {
>  		struct page *newpage = migrate_pfn_to_page(dst_pfns[i]);
> @@ -1093,12 +1141,16 @@ static void __migrate_device_pages(unsigned long *src_pfns,
>  				(!(dst_pfns[i] & MIGRATE_PFN_COMPOUND))) {
>  				nr = migrate_vma_nr_pages(&src_pfns[i]);
>  				src_pfns[i] &= ~MIGRATE_PFN_COMPOUND;
> -				src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
> -				goto next;
> +			} else {
> +				nr = 1;
>  			}
>
> -			migrate_vma_insert_page(migrate, addr, &dst_pfns[i],
> -						&src_pfns[i]);
> +			for (j = 0; j < nr && i + j < npages; j++) {
> +				src_pfns[i+j] |= MIGRATE_PFN_MIGRATE;
> +				migrate_vma_insert_page(migrate,
> +					addr + j * PAGE_SIZE,
> +					&dst_pfns[i+j], &src_pfns[i+j]);
> +			}
>  			goto next;
>  		}
>
> @@ -1120,7 +1172,13 @@ static void __migrate_device_pages(unsigned long *src_pfns,
>  							 MIGRATE_PFN_COMPOUND);
>  					goto next;
>  				}
> -				src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
> +				nr = 1 << folio_order(folio);
> +				addr = migrate->start + i * PAGE_SIZE;
> +				if (migrate_vma_split_unmapped_folio(migrate, i, addr, folio)) {
> +					src_pfns[i] &= ~(MIGRATE_PFN_MIGRATE |
> +							 MIGRATE_PFN_COMPOUND);
> +					goto next;
> +				}
>  			} else if ((src_pfns[i] & MIGRATE_PFN_MIGRATE) &&
>  				(dst_pfns[i] & MIGRATE_PFN_COMPOUND) &&
>  				!(src_pfns[i] & MIGRATE_PFN_COMPOUND)) {
> @@ -1156,11 +1214,16 @@ static void __migrate_device_pages(unsigned long *src_pfns,
>
>  		if (migrate && migrate->fault_page == page)
>  			extra_cnt = 1;
> -		r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt);
> -		if (r)
> -			src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
> -		else
> -			folio_migrate_flags(newfolio, folio);
> +		for (j = 0; j < nr && i + j < npages; j++) {
> +			folio = page_folio(migrate_pfn_to_page(src_pfns[i+j]));
> +			newfolio = page_folio(migrate_pfn_to_page(dst_pfns[i+j]));
> +
> +			r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt);
> +			if (r)
> +				src_pfns[i+j] &= ~MIGRATE_PFN_MIGRATE;
> +			else
> +				folio_migrate_flags(newfolio, folio);
> +		}
>  next:
>  		i += nr;
>  	}
> -- 
> 2.51.0


--
Best Regards,
Yan, Zi

Powered by blists - more mailing lists