Re: [v3 03/11] mm/migrate_device: THP migration of zone device pages

["Mika Penttilä" <mpenttil@redhat.com>]

Hi,

On 8/12/25 05:40, Balbir Singh wrote:

> MIGRATE_VMA_SELECT_COMPOUND will be used to select THP pages during
> migrate_vma_setup() and MIGRATE_PFN_COMPOUND will make migrating
> device pages as compound pages during device pfn migration.
>
> migrate_device code paths go through the collect, setup
> and finalize phases of migration.
>
> The entries in src and dst arrays passed to these functions still
> remain at a PAGE_SIZE granularity. When a compound page is passed,
> the first entry has the PFN along with MIGRATE_PFN_COMPOUND
> and other flags set (MIGRATE_PFN_MIGRATE, MIGRATE_PFN_VALID), the
> remaining entries (HPAGE_PMD_NR - 1) are filled with 0's. This
> representation allows for the compound page to be split into smaller
> page sizes.
>
> migrate_vma_collect_hole(), migrate_vma_collect_pmd() are now THP
> page aware. Two new helper functions migrate_vma_collect_huge_pmd()
> and migrate_vma_insert_huge_pmd_page() have been added.
>
> migrate_vma_collect_huge_pmd() can collect THP pages, but if for
> some reason this fails, there is fallback support to split the folio
> and migrate it.
>
> migrate_vma_insert_huge_pmd_page() closely follows the logic of
> migrate_vma_insert_page()
>
> Support for splitting pages as needed for migration will follow in
> later patches in this series.
>
> 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/migrate.h |   2 +
>  mm/migrate_device.c     | 457 ++++++++++++++++++++++++++++++++++------
>  2 files changed, 396 insertions(+), 63 deletions(-)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index acadd41e0b5c..d9cef0819f91 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -129,6 +129,7 @@ static inline int migrate_misplaced_folio(struct folio *folio, int node)
>  #define MIGRATE_PFN_VALID	(1UL << 0)
>  #define MIGRATE_PFN_MIGRATE	(1UL << 1)
>  #define MIGRATE_PFN_WRITE	(1UL << 3)
> +#define MIGRATE_PFN_COMPOUND	(1UL << 4)
>  #define MIGRATE_PFN_SHIFT	6
>  
>  static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
> @@ -147,6 +148,7 @@ enum migrate_vma_direction {
>  	MIGRATE_VMA_SELECT_SYSTEM = 1 << 0,
>  	MIGRATE_VMA_SELECT_DEVICE_PRIVATE = 1 << 1,
>  	MIGRATE_VMA_SELECT_DEVICE_COHERENT = 1 << 2,
> +	MIGRATE_VMA_SELECT_COMPOUND = 1 << 3,
>  };
>  
>  struct migrate_vma {
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 0ed337f94fcd..6621bba62710 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -14,6 +14,7 @@
>  #include <linux/pagewalk.h>
>  #include <linux/rmap.h>
>  #include <linux/swapops.h>
> +#include <asm/pgalloc.h>
>  #include <asm/tlbflush.h>
>  #include "internal.h"
>  
> @@ -44,6 +45,23 @@ static int migrate_vma_collect_hole(unsigned long start,
>  	if (!vma_is_anonymous(walk->vma))
>  		return migrate_vma_collect_skip(start, end, walk);
>  
> +	if (thp_migration_supported() &&
> +		(migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
> +		(IS_ALIGNED(start, HPAGE_PMD_SIZE) &&
> +		 IS_ALIGNED(end, HPAGE_PMD_SIZE))) {
> +		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE |
> +						MIGRATE_PFN_COMPOUND;
> +		migrate->dst[migrate->npages] = 0;
> +		migrate->npages++;
> +		migrate->cpages++;
> +
> +		/*
> +		 * Collect the remaining entries as holes, in case we
> +		 * need to split later
> +		 */
> +		return migrate_vma_collect_skip(start + PAGE_SIZE, end, walk);
> +	}
> +
>  	for (addr = start; addr < end; addr += PAGE_SIZE) {
>  		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
>  		migrate->dst[migrate->npages] = 0;
> @@ -54,57 +72,151 @@ static int migrate_vma_collect_hole(unsigned long start,
>  	return 0;
>  }
>  
> -static int migrate_vma_collect_pmd(pmd_t *pmdp,
> -				   unsigned long start,
> -				   unsigned long end,
> -				   struct mm_walk *walk)
> +/**
> + * migrate_vma_collect_huge_pmd - collect THP pages without splitting the
> + * folio for device private pages.
> + * @pmdp: pointer to pmd entry
> + * @start: start address of the range for migration
> + * @end: end address of the range for migration
> + * @walk: mm_walk callback structure
> + *
> + * Collect the huge pmd entry at @pmdp for migration and set the
> + * MIGRATE_PFN_COMPOUND flag in the migrate src entry to indicate that
> + * migration will occur at HPAGE_PMD granularity
> + */
> +static int migrate_vma_collect_huge_pmd(pmd_t *pmdp, unsigned long start,
> +					unsigned long end, struct mm_walk *walk,
> +					struct folio *fault_folio)
>  {
> +	struct mm_struct *mm = walk->mm;
> +	struct folio *folio;
>  	struct migrate_vma *migrate = walk->private;
> -	struct folio *fault_folio = migrate->fault_page ?
> -		page_folio(migrate->fault_page) : NULL;
> -	struct vm_area_struct *vma = walk->vma;
> -	struct mm_struct *mm = vma->vm_mm;
> -	unsigned long addr = start, unmapped = 0;
>  	spinlock_t *ptl;
> -	pte_t *ptep;
> +	swp_entry_t entry;
> +	int ret;
> +	unsigned long write = 0;
>  
> -again:
> -	if (pmd_none(*pmdp))
> +	ptl = pmd_lock(mm, pmdp);
> +	if (pmd_none(*pmdp)) {
> +		spin_unlock(ptl);
>  		return migrate_vma_collect_hole(start, end, -1, walk);
> +	}
>  
>  	if (pmd_trans_huge(*pmdp)) {
> -		struct folio *folio;
> -
> -		ptl = pmd_lock(mm, pmdp);
> -		if (unlikely(!pmd_trans_huge(*pmdp))) {
> +		if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM)) {
>  			spin_unlock(ptl);
> -			goto again;
> +			return migrate_vma_collect_skip(start, end, walk);
>  		}
>  
>  		folio = pmd_folio(*pmdp);
>  		if (is_huge_zero_folio(folio)) {
>  			spin_unlock(ptl);
> -			split_huge_pmd(vma, pmdp, addr);
> -		} else {
> -			int ret;
> +			return migrate_vma_collect_hole(start, end, -1, walk);
> +		}
> +		if (pmd_write(*pmdp))
> +			write = MIGRATE_PFN_WRITE;
> +	} else if (!pmd_present(*pmdp)) {
> +		entry = pmd_to_swp_entry(*pmdp);
> +		folio = pfn_swap_entry_folio(entry);
> +
> +		if (!is_device_private_entry(entry) ||
> +			!(migrate->flags & MIGRATE_VMA_SELECT_DEVICE_PRIVATE) ||
> +			(folio->pgmap->owner != migrate->pgmap_owner)) {
> +			spin_unlock(ptl);
> +			return migrate_vma_collect_skip(start, end, walk);
> +		}
>  
> -			folio_get(folio);
> +		if (is_migration_entry(entry)) {
> +			migration_entry_wait_on_locked(entry, ptl);
>  			spin_unlock(ptl);
> -			/* FIXME: we don't expect THP for fault_folio */
> -			if (WARN_ON_ONCE(fault_folio == folio))
> -				return migrate_vma_collect_skip(start, end,
> -								walk);
> -			if (unlikely(!folio_trylock(folio)))
> -				return migrate_vma_collect_skip(start, end,
> -								walk);
> -			ret = split_folio(folio);
> -			if (fault_folio != folio)
> -				folio_unlock(folio);
> -			folio_put(folio);
> -			if (ret)
> -				return migrate_vma_collect_skip(start, end,
> -								walk);
> +			return -EAGAIN;
>  		}
> +
> +		if (is_writable_device_private_entry(entry))
> +			write = MIGRATE_PFN_WRITE;
> +	} else {
> +		spin_unlock(ptl);
> +		return -EAGAIN;
> +	}
> +
> +	folio_get(folio);
> +	if (folio != fault_folio && unlikely(!folio_trylock(folio))) {
> +		spin_unlock(ptl);
> +		folio_put(folio);
> +		return migrate_vma_collect_skip(start, end, walk);
> +	}
> +
> +	if (thp_migration_supported() &&
> +		(migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
> +		(IS_ALIGNED(start, HPAGE_PMD_SIZE) &&
> +		 IS_ALIGNED(end, HPAGE_PMD_SIZE))) {
> +
> +		struct page_vma_mapped_walk pvmw = {
> +			.ptl = ptl,
> +			.address = start,
> +			.pmd = pmdp,
> +			.vma = walk->vma,
> +		};
> +
> +		unsigned long pfn = page_to_pfn(folio_page(folio, 0));
> +
> +		migrate->src[migrate->npages] = migrate_pfn(pfn) | write
> +						| MIGRATE_PFN_MIGRATE
> +						| MIGRATE_PFN_COMPOUND;
> +		migrate->dst[migrate->npages++] = 0;
> +		migrate->cpages++;
> +		ret = set_pmd_migration_entry(&pvmw, folio_page(folio, 0));
> +		if (ret) {
> +			migrate->npages--;
> +			migrate->cpages--;
> +			migrate->src[migrate->npages] = 0;
> +			migrate->dst[migrate->npages] = 0;
> +			goto fallback;
> +		}
> +		migrate_vma_collect_skip(start + PAGE_SIZE, end, walk);
> +		spin_unlock(ptl);
> +		return 0;
> +	}
> +
> +fallback:
> +	spin_unlock(ptl);
> +	if (!folio_test_large(folio))
> +		goto done;
> +	ret = split_folio(folio);
> +	if (fault_folio != folio)
> +		folio_unlock(folio);
> +	folio_put(folio);
> +	if (ret)
> +		return migrate_vma_collect_skip(start, end, walk);
> +	if (pmd_none(pmdp_get_lockless(pmdp)))
> +		return migrate_vma_collect_hole(start, end, -1, walk);
> +
> +done:
> +	return -ENOENT;
> +}
> +
> +static int migrate_vma_collect_pmd(pmd_t *pmdp,
> +				   unsigned long start,
> +				   unsigned long end,
> +				   struct mm_walk *walk)
> +{
> +	struct migrate_vma *migrate = walk->private;
> +	struct vm_area_struct *vma = walk->vma;
> +	struct mm_struct *mm = vma->vm_mm;
> +	unsigned long addr = start, unmapped = 0;
> +	spinlock_t *ptl;
> +	struct folio *fault_folio = migrate->fault_page ?
> +		page_folio(migrate->fault_page) : NULL;
> +	pte_t *ptep;
> +
> +again:
> +	if (pmd_trans_huge(*pmdp) || !pmd_present(*pmdp)) {
> +		int ret = migrate_vma_collect_huge_pmd(pmdp, start, end, walk, fault_folio);
> +
> +		if (ret == -EAGAIN)
> +			goto again;
> +		if (ret == 0)
> +			return 0;
>  	}
>  
>  	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> @@ -222,8 +334,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
>  		}
>  
> -		/* FIXME support THP */
> -		if (!page || !page->mapping || PageTransCompound(page)) {
> +		if (!page || !page->mapping) {
>  			mpfn = 0;
>  			goto next;
>  		}
> @@ -394,14 +505,6 @@ static bool migrate_vma_check_page(struct page *page, struct page *fault_page)
>  	 */
>  	int extra = 1 + (page == fault_page);
>  
> -	/*
> -	 * FIXME support THP (transparent huge page), it is bit more complex to
> -	 * check them than regular pages, because they can be mapped with a pmd
> -	 * or with a pte (split pte mapping).
> -	 */
> -	if (folio_test_large(folio))
> -		return false;
> -

You cannot remove this check unless support normal mTHP folios migrate to device, 
which I think this series doesn't do, but maybe should?

--Mika