Re: [PATCH 13/27] mm: move the migrate_vma_* device migration code into it's own file

[Alistair Popple]

I got the following build error:

/data/source/linux/mm/migrate_device.c: In function ‘migrate_vma_collect_pmd’:
/data/source/linux/mm/migrate_device.c:242:3: error: implicit declaration of function ‘flush_tlb_range’; did you mean ‘flush_pmd_tlb_range’? [-Werror=implicit-function-declaration]
  242 |   flush_tlb_range(walk->vma, start, end);
      |   ^~~~~~~~~~~~~~~
      |   flush_pmd_tlb_range

Including asm/tlbflush.h in migrate_device.c fixed it for me.

On Thursday, 10 February 2022 6:28:14 PM AEDT Christoph Hellwig wrote:
> Split the code used to migrate to and from ZONE_DEVICE memory from
> migrate.c into a new file.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  mm/Kconfig          |   3 +
>  mm/Makefile         |   1 +
>  mm/migrate.c        | 753 -------------------------------------------
>  mm/migrate_device.c | 765 ++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 769 insertions(+), 753 deletions(-)
>  create mode 100644 mm/migrate_device.c
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index a1901ae6d06293..6391d8d3a616f3 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -249,6 +249,9 @@ config MIGRATION
>  	  pages as migration can relocate pages to satisfy a huge page
>  	  allocation instead of reclaiming.
>  
> +config DEVICE_MIGRATION
> +	def_bool MIGRATION && DEVICE_PRIVATE
> +
>  config ARCH_ENABLE_HUGEPAGE_MIGRATION
>  	bool
>  
> diff --git a/mm/Makefile b/mm/Makefile
> index 70d4309c9ce338..4cc13f3179a518 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -92,6 +92,7 @@ obj-$(CONFIG_KFENCE) += kfence/
>  obj-$(CONFIG_FAILSLAB) += failslab.o
>  obj-$(CONFIG_MEMTEST)		+= memtest.o
>  obj-$(CONFIG_MIGRATION) += migrate.o
> +obj-$(CONFIG_DEVICE_MIGRATION) += migrate_device.o
>  obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o khugepaged.o
>  obj-$(CONFIG_PAGE_COUNTER) += page_counter.o
>  obj-$(CONFIG_MEMCG) += memcontrol.o vmpressure.o
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 746e1230886ddb..c31d04b46a5e17 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -38,12 +38,10 @@
>  #include <linux/hugetlb.h>
>  #include <linux/hugetlb_cgroup.h>
>  #include <linux/gfp.h>
> -#include <linux/pagewalk.h>
>  #include <linux/pfn_t.h>
>  #include <linux/memremap.h>
>  #include <linux/userfaultfd_k.h>
>  #include <linux/balloon_compaction.h>
> -#include <linux/mmu_notifier.h>
>  #include <linux/page_idle.h>
>  #include <linux/page_owner.h>
>  #include <linux/sched/mm.h>
> @@ -2125,757 +2123,6 @@ int migrate_misplaced_page(struct page *page, struct vm_area_struct *vma,
>  #endif /* CONFIG_NUMA_BALANCING */
>  #endif /* CONFIG_NUMA */
>  
> -#ifdef CONFIG_DEVICE_PRIVATE
> -static int migrate_vma_collect_skip(unsigned long start,
> -				    unsigned long end,
> -				    struct mm_walk *walk)
> -{
> -	struct migrate_vma *migrate = walk->private;
> -	unsigned long addr;
> -
> -	for (addr = start; addr < end; addr += PAGE_SIZE) {
> -		migrate->dst[migrate->npages] = 0;
> -		migrate->src[migrate->npages++] = 0;
> -	}
> -
> -	return 0;
> -}
> -
> -static int migrate_vma_collect_hole(unsigned long start,
> -				    unsigned long end,
> -				    __always_unused int depth,
> -				    struct mm_walk *walk)
> -{
> -	struct migrate_vma *migrate = walk->private;
> -	unsigned long addr;
> -
> -	/* Only allow populating anonymous memory. */
> -	if (!vma_is_anonymous(walk->vma))
> -		return migrate_vma_collect_skip(start, end, walk);
> -
> -	for (addr = start; addr < end; addr += PAGE_SIZE) {
> -		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
> -		migrate->dst[migrate->npages] = 0;
> -		migrate->npages++;
> -		migrate->cpages++;
> -	}
> -
> -	return 0;
> -}
> -
> -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;
> -	pte_t *ptep;
> -
> -again:
> -	if (pmd_none(*pmdp))
> -		return migrate_vma_collect_hole(start, end, -1, walk);
> -
> -	if (pmd_trans_huge(*pmdp)) {
> -		struct page *page;
> -
> -		ptl = pmd_lock(mm, pmdp);
> -		if (unlikely(!pmd_trans_huge(*pmdp))) {
> -			spin_unlock(ptl);
> -			goto again;
> -		}
> -
> -		page = pmd_page(*pmdp);
> -		if (is_huge_zero_page(page)) {
> -			spin_unlock(ptl);
> -			split_huge_pmd(vma, pmdp, addr);
> -			if (pmd_trans_unstable(pmdp))
> -				return migrate_vma_collect_skip(start, end,
> -								walk);
> -		} else {
> -			int ret;
> -
> -			get_page(page);
> -			spin_unlock(ptl);
> -			if (unlikely(!trylock_page(page)))
> -				return migrate_vma_collect_skip(start, end,
> -								walk);
> -			ret = split_huge_page(page);
> -			unlock_page(page);
> -			put_page(page);
> -			if (ret)
> -				return migrate_vma_collect_skip(start, end,
> -								walk);
> -			if (pmd_none(*pmdp))
> -				return migrate_vma_collect_hole(start, end, -1,
> -								walk);
> -		}
> -	}
> -
> -	if (unlikely(pmd_bad(*pmdp)))
> -		return migrate_vma_collect_skip(start, end, walk);
> -
> -	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> -	arch_enter_lazy_mmu_mode();
> -
> -	for (; addr < end; addr += PAGE_SIZE, ptep++) {
> -		unsigned long mpfn = 0, pfn;
> -		struct page *page;
> -		swp_entry_t entry;
> -		pte_t pte;
> -
> -		pte = *ptep;
> -
> -		if (pte_none(pte)) {
> -			if (vma_is_anonymous(vma)) {
> -				mpfn = MIGRATE_PFN_MIGRATE;
> -				migrate->cpages++;
> -			}
> -			goto next;
> -		}
> -
> -		if (!pte_present(pte)) {
> -			/*
> -			 * Only care about unaddressable device page special
> -			 * page table entry. Other special swap entries are not
> -			 * migratable, and we ignore regular swapped page.
> -			 */
> -			entry = pte_to_swp_entry(pte);
> -			if (!is_device_private_entry(entry))
> -				goto next;
> -
> -			page = pfn_swap_entry_to_page(entry);
> -			if (!(migrate->flags &
> -				MIGRATE_VMA_SELECT_DEVICE_PRIVATE) ||
> -			    page->pgmap->owner != migrate->pgmap_owner)
> -				goto next;
> -
> -			mpfn = migrate_pfn(page_to_pfn(page)) |
> -					MIGRATE_PFN_MIGRATE;
> -			if (is_writable_device_private_entry(entry))
> -				mpfn |= MIGRATE_PFN_WRITE;
> -		} else {
> -			if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM))
> -				goto next;
> -			pfn = pte_pfn(pte);
> -			if (is_zero_pfn(pfn)) {
> -				mpfn = MIGRATE_PFN_MIGRATE;
> -				migrate->cpages++;
> -				goto next;
> -			}
> -			page = vm_normal_page(migrate->vma, addr, pte);
> -			mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
> -			mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
> -		}
> -
> -		/* FIXME support THP */
> -		if (!page || !page->mapping || PageTransCompound(page)) {
> -			mpfn = 0;
> -			goto next;
> -		}
> -
> -		/*
> -		 * By getting a reference on the page we pin it and that blocks
> -		 * any kind of migration. Side effect is that it "freezes" the
> -		 * pte.
> -		 *
> -		 * We drop this reference after isolating the page from the lru
> -		 * for non device page (device page are not on the lru and thus
> -		 * can't be dropped from it).
> -		 */
> -		get_page(page);
> -
> -		/*
> -		 * Optimize for the common case where page is only mapped once
> -		 * in one process. If we can lock the page, then we can safely
> -		 * set up a special migration page table entry now.
> -		 */
> -		if (trylock_page(page)) {
> -			pte_t swp_pte;
> -
> -			migrate->cpages++;
> -			ptep_get_and_clear(mm, addr, ptep);
> -
> -			/* Setup special migration page table entry */
> -			if (mpfn & MIGRATE_PFN_WRITE)
> -				entry = make_writable_migration_entry(
> -							page_to_pfn(page));
> -			else
> -				entry = make_readable_migration_entry(
> -							page_to_pfn(page));
> -			swp_pte = swp_entry_to_pte(entry);
> -			if (pte_present(pte)) {
> -				if (pte_soft_dirty(pte))
> -					swp_pte = pte_swp_mksoft_dirty(swp_pte);
> -				if (pte_uffd_wp(pte))
> -					swp_pte = pte_swp_mkuffd_wp(swp_pte);
> -			} else {
> -				if (pte_swp_soft_dirty(pte))
> -					swp_pte = pte_swp_mksoft_dirty(swp_pte);
> -				if (pte_swp_uffd_wp(pte))
> -					swp_pte = pte_swp_mkuffd_wp(swp_pte);
> -			}
> -			set_pte_at(mm, addr, ptep, swp_pte);
> -
> -			/*
> -			 * This is like regular unmap: we remove the rmap and
> -			 * drop page refcount. Page won't be freed, as we took
> -			 * a reference just above.
> -			 */
> -			page_remove_rmap(page, false);
> -			put_page(page);
> -
> -			if (pte_present(pte))
> -				unmapped++;
> -		} else {
> -			put_page(page);
> -			mpfn = 0;
> -		}
> -
> -next:
> -		migrate->dst[migrate->npages] = 0;
> -		migrate->src[migrate->npages++] = mpfn;
> -	}
> -	arch_leave_lazy_mmu_mode();
> -	pte_unmap_unlock(ptep - 1, ptl);
> -
> -	/* Only flush the TLB if we actually modified any entries */
> -	if (unmapped)
> -		flush_tlb_range(walk->vma, start, end);
> -
> -	return 0;
> -}
> -
> -static const struct mm_walk_ops migrate_vma_walk_ops = {
> -	.pmd_entry		= migrate_vma_collect_pmd,
> -	.pte_hole		= migrate_vma_collect_hole,
> -};
> -
> -/*
> - * migrate_vma_collect() - collect pages over a range of virtual addresses
> - * @migrate: migrate struct containing all migration information
> - *
> - * This will walk the CPU page table. For each virtual address backed by a
> - * valid page, it updates the src array and takes a reference on the page, in
> - * order to pin the page until we lock it and unmap it.
> - */
> -static void migrate_vma_collect(struct migrate_vma *migrate)
> -{
> -	struct mmu_notifier_range range;
> -
> -	/*
> -	 * Note that the pgmap_owner is passed to the mmu notifier callback so
> -	 * that the registered device driver can skip invalidating device
> -	 * private page mappings that won't be migrated.
> -	 */
> -	mmu_notifier_range_init_owner(&range, MMU_NOTIFY_MIGRATE, 0,
> -		migrate->vma, migrate->vma->vm_mm, migrate->start, migrate->end,
> -		migrate->pgmap_owner);
> -	mmu_notifier_invalidate_range_start(&range);
> -
> -	walk_page_range(migrate->vma->vm_mm, migrate->start, migrate->end,
> -			&migrate_vma_walk_ops, migrate);
> -
> -	mmu_notifier_invalidate_range_end(&range);
> -	migrate->end = migrate->start + (migrate->npages << PAGE_SHIFT);
> -}
> -
> -/*
> - * migrate_vma_check_page() - check if page is pinned or not
> - * @page: struct page to check
> - *
> - * Pinned pages cannot be migrated. This is the same test as in
> - * folio_migrate_mapping(), except that here we allow migration of a
> - * ZONE_DEVICE page.
> - */
> -static bool migrate_vma_check_page(struct page *page)
> -{
> -	/*
> -	 * One extra ref because caller holds an extra reference, either from
> -	 * isolate_lru_page() for a regular page, or migrate_vma_collect() for
> -	 * a device page.
> -	 */
> -	int extra = 1;
> -
> -	/*
> -	 * 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 (PageCompound(page))
> -		return false;
> -
> -	/* Page from ZONE_DEVICE have one extra reference */
> -	if (is_zone_device_page(page))
> -		extra++;
> -
> -	/* For file back page */
> -	if (page_mapping(page))
> -		extra += 1 + page_has_private(page);
> -
> -	if ((page_count(page) - extra) > page_mapcount(page))
> -		return false;
> -
> -	return true;
> -}
> -
> -/*
> - * migrate_vma_unmap() - replace page mapping with special migration pte entry
> - * @migrate: migrate struct containing all migration information
> - *
> - * Isolate pages from the LRU and replace mappings (CPU page table pte) with a
> - * special migration pte entry and check if it has been pinned. Pinned pages are
> - * restored because we cannot migrate them.
> - *
> - * This is the last step before we call the device driver callback to allocate
> - * destination memory and copy contents of original page over to new page.
> - */
> -static void migrate_vma_unmap(struct migrate_vma *migrate)
> -{
> -	const unsigned long npages = migrate->npages;
> -	unsigned long i, restore = 0;
> -	bool allow_drain = true;
> -
> -	lru_add_drain();
> -
> -	for (i = 0; i < npages; i++) {
> -		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> -
> -		if (!page)
> -			continue;
> -
> -		/* ZONE_DEVICE pages are not on LRU */
> -		if (!is_zone_device_page(page)) {
> -			if (!PageLRU(page) && allow_drain) {
> -				/* Drain CPU's pagevec */
> -				lru_add_drain_all();
> -				allow_drain = false;
> -			}
> -
> -			if (isolate_lru_page(page)) {
> -				migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> -				migrate->cpages--;
> -				restore++;
> -				continue;
> -			}
> -
> -			/* Drop the reference we took in collect */
> -			put_page(page);
> -		}
> -
> -		if (page_mapped(page))
> -			try_to_migrate(page, 0);
> -
> -		if (page_mapped(page) || !migrate_vma_check_page(page)) {
> -			if (!is_zone_device_page(page)) {
> -				get_page(page);
> -				putback_lru_page(page);
> -			}
> -
> -			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> -			migrate->cpages--;
> -			restore++;
> -			continue;
> -		}
> -	}
> -
> -	for (i = 0; i < npages && restore; i++) {
> -		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> -
> -		if (!page || (migrate->src[i] & MIGRATE_PFN_MIGRATE))
> -			continue;
> -
> -		remove_migration_ptes(page, page, false);
> -
> -		migrate->src[i] = 0;
> -		unlock_page(page);
> -		put_page(page);
> -		restore--;
> -	}
> -}
> -
> -/**
> - * migrate_vma_setup() - prepare to migrate a range of memory
> - * @args: contains the vma, start, and pfns arrays for the migration
> - *
> - * Returns: negative errno on failures, 0 when 0 or more pages were migrated
> - * without an error.
> - *
> - * Prepare to migrate a range of memory virtual address range by collecting all
> - * the pages backing each virtual address in the range, saving them inside the
> - * src array.  Then lock those pages and unmap them. Once the pages are locked
> - * and unmapped, check whether each page is pinned or not.  Pages that aren't
> - * pinned have the MIGRATE_PFN_MIGRATE flag set (by this function) in the
> - * corresponding src array entry.  Then restores any pages that are pinned, by
> - * remapping and unlocking those pages.
> - *
> - * The caller should then allocate destination memory and copy source memory to
> - * it for all those entries (ie with MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE
> - * flag set).  Once these are allocated and copied, the caller must update each
> - * corresponding entry in the dst array with the pfn value of the destination
> - * page and with MIGRATE_PFN_VALID. Destination pages must be locked via
> - * lock_page().
> - *
> - * Note that the caller does not have to migrate all the pages that are marked
> - * with MIGRATE_PFN_MIGRATE flag in src array unless this is a migration from
> - * device memory to system memory.  If the caller cannot migrate a device page
> - * back to system memory, then it must return VM_FAULT_SIGBUS, which has severe
> - * consequences for the userspace process, so it must be avoided if at all
> - * possible.
> - *
> - * For empty entries inside CPU page table (pte_none() or pmd_none() is true) we
> - * do set MIGRATE_PFN_MIGRATE flag inside the corresponding source array thus
> - * allowing the caller to allocate device memory for those unbacked virtual
> - * addresses.  For this the caller simply has to allocate device memory and
> - * properly set the destination entry like for regular migration.  Note that
> - * this can still fail, and thus inside the device driver you must check if the
> - * migration was successful for those entries after calling migrate_vma_pages(),
> - * just like for regular migration.
> - *
> - * After that, the callers must call migrate_vma_pages() to go over each entry
> - * in the src array that has the MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag
> - * set. If the corresponding entry in dst array has MIGRATE_PFN_VALID flag set,
> - * then migrate_vma_pages() to migrate struct page information from the source
> - * struct page to the destination struct page.  If it fails to migrate the
> - * struct page information, then it clears the MIGRATE_PFN_MIGRATE flag in the
> - * src array.
> - *
> - * At this point all successfully migrated pages have an entry in the src
> - * array with MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag set and the dst
> - * array entry with MIGRATE_PFN_VALID flag set.
> - *
> - * Once migrate_vma_pages() returns the caller may inspect which pages were
> - * successfully migrated, and which were not.  Successfully migrated pages will
> - * have the MIGRATE_PFN_MIGRATE flag set for their src array entry.
> - *
> - * It is safe to update device page table after migrate_vma_pages() because
> - * both destination and source page are still locked, and the mmap_lock is held
> - * in read mode (hence no one can unmap the range being migrated).
> - *
> - * Once the caller is done cleaning up things and updating its page table (if it
> - * chose to do so, this is not an obligation) it finally calls
> - * migrate_vma_finalize() to update the CPU page table to point to new pages
> - * for successfully migrated pages or otherwise restore the CPU page table to
> - * point to the original source pages.
> - */
> -int migrate_vma_setup(struct migrate_vma *args)
> -{
> -	long nr_pages = (args->end - args->start) >> PAGE_SHIFT;
> -
> -	args->start &= PAGE_MASK;
> -	args->end &= PAGE_MASK;
> -	if (!args->vma || is_vm_hugetlb_page(args->vma) ||
> -	    (args->vma->vm_flags & VM_SPECIAL) || vma_is_dax(args->vma))
> -		return -EINVAL;
> -	if (nr_pages <= 0)
> -		return -EINVAL;
> -	if (args->start < args->vma->vm_start ||
> -	    args->start >= args->vma->vm_end)
> -		return -EINVAL;
> -	if (args->end <= args->vma->vm_start || args->end > args->vma->vm_end)
> -		return -EINVAL;
> -	if (!args->src || !args->dst)
> -		return -EINVAL;
> -
> -	memset(args->src, 0, sizeof(*args->src) * nr_pages);
> -	args->cpages = 0;
> -	args->npages = 0;
> -
> -	migrate_vma_collect(args);
> -
> -	if (args->cpages)
> -		migrate_vma_unmap(args);
> -
> -	/*
> -	 * At this point pages are locked and unmapped, and thus they have
> -	 * stable content and can safely be copied to destination memory that
> -	 * is allocated by the drivers.
> -	 */
> -	return 0;
> -
> -}
> -EXPORT_SYMBOL(migrate_vma_setup);
> -
> -/*
> - * This code closely matches the code in:
> - *   __handle_mm_fault()
> - *     handle_pte_fault()
> - *       do_anonymous_page()
> - * to map in an anonymous zero page but the struct page will be a ZONE_DEVICE
> - * private page.
> - */
> -static void migrate_vma_insert_page(struct migrate_vma *migrate,
> -				    unsigned long addr,
> -				    struct page *page,
> -				    unsigned long *src)
> -{
> -	struct vm_area_struct *vma = migrate->vma;
> -	struct mm_struct *mm = vma->vm_mm;
> -	bool flush = false;
> -	spinlock_t *ptl;
> -	pte_t entry;
> -	pgd_t *pgdp;
> -	p4d_t *p4dp;
> -	pud_t *pudp;
> -	pmd_t *pmdp;
> -	pte_t *ptep;
> -
> -	/* Only allow populating anonymous memory */
> -	if (!vma_is_anonymous(vma))
> -		goto abort;
> -
> -	pgdp = pgd_offset(mm, addr);
> -	p4dp = p4d_alloc(mm, pgdp, addr);
> -	if (!p4dp)
> -		goto abort;
> -	pudp = pud_alloc(mm, p4dp, addr);
> -	if (!pudp)
> -		goto abort;
> -	pmdp = pmd_alloc(mm, pudp, addr);
> -	if (!pmdp)
> -		goto abort;
> -
> -	if (pmd_trans_huge(*pmdp) || pmd_devmap(*pmdp))
> -		goto abort;
> -
> -	/*
> -	 * Use pte_alloc() instead of pte_alloc_map().  We can't run
> -	 * pte_offset_map() on pmds where a huge pmd might be created
> -	 * from a different thread.
> -	 *
> -	 * pte_alloc_map() is safe to use under mmap_write_lock(mm) or when
> -	 * parallel threads are excluded by other means.
> -	 *
> -	 * Here we only have mmap_read_lock(mm).
> -	 */
> -	if (pte_alloc(mm, pmdp))
> -		goto abort;
> -
> -	/* See the comment in pte_alloc_one_map() */
> -	if (unlikely(pmd_trans_unstable(pmdp)))
> -		goto abort;
> -
> -	if (unlikely(anon_vma_prepare(vma)))
> -		goto abort;
> -	if (mem_cgroup_charge(page_folio(page), vma->vm_mm, GFP_KERNEL))
> -		goto abort;
> -
> -	/*
> -	 * The memory barrier inside __SetPageUptodate makes sure that
> -	 * preceding stores to the page contents become visible before
> -	 * the set_pte_at() write.
> -	 */
> -	__SetPageUptodate(page);
> -
> -	if (is_device_private_page(page)) {
> -		swp_entry_t swp_entry;
> -
> -		if (vma->vm_flags & VM_WRITE)
> -			swp_entry = make_writable_device_private_entry(
> -						page_to_pfn(page));
> -		else
> -			swp_entry = make_readable_device_private_entry(
> -						page_to_pfn(page));
> -		entry = swp_entry_to_pte(swp_entry);
> -	} else {
> -		/*
> -		 * For now we only support migrating to un-addressable device
> -		 * memory.
> -		 */
> -		if (is_zone_device_page(page)) {
> -			pr_warn_once("Unsupported ZONE_DEVICE page type.\n");
> -			goto abort;
> -		}
> -		entry = mk_pte(page, vma->vm_page_prot);
> -		if (vma->vm_flags & VM_WRITE)
> -			entry = pte_mkwrite(pte_mkdirty(entry));
> -	}
> -
> -	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> -
> -	if (check_stable_address_space(mm))
> -		goto unlock_abort;
> -
> -	if (pte_present(*ptep)) {
> -		unsigned long pfn = pte_pfn(*ptep);
> -
> -		if (!is_zero_pfn(pfn))
> -			goto unlock_abort;
> -		flush = true;
> -	} else if (!pte_none(*ptep))
> -		goto unlock_abort;
> -
> -	/*
> -	 * Check for userfaultfd but do not deliver the fault. Instead,
> -	 * just back off.
> -	 */
> -	if (userfaultfd_missing(vma))
> -		goto unlock_abort;
> -
> -	inc_mm_counter(mm, MM_ANONPAGES);
> -	page_add_new_anon_rmap(page, vma, addr, false);
> -	if (!is_zone_device_page(page))
> -		lru_cache_add_inactive_or_unevictable(page, vma);
> -	get_page(page);
> -
> -	if (flush) {
> -		flush_cache_page(vma, addr, pte_pfn(*ptep));
> -		ptep_clear_flush_notify(vma, addr, ptep);
> -		set_pte_at_notify(mm, addr, ptep, entry);
> -		update_mmu_cache(vma, addr, ptep);
> -	} else {
> -		/* No need to invalidate - it was non-present before */
> -		set_pte_at(mm, addr, ptep, entry);
> -		update_mmu_cache(vma, addr, ptep);
> -	}
> -
> -	pte_unmap_unlock(ptep, ptl);
> -	*src = MIGRATE_PFN_MIGRATE;
> -	return;
> -
> -unlock_abort:
> -	pte_unmap_unlock(ptep, ptl);
> -abort:
> -	*src &= ~MIGRATE_PFN_MIGRATE;
> -}
> -
> -/**
> - * migrate_vma_pages() - migrate meta-data from src page to dst page
> - * @migrate: migrate struct containing all migration information
> - *
> - * This migrates struct page meta-data from source struct page to destination
> - * struct page. This effectively finishes the migration from source page to the
> - * destination page.
> - */
> -void migrate_vma_pages(struct migrate_vma *migrate)
> -{
> -	const unsigned long npages = migrate->npages;
> -	const unsigned long start = migrate->start;
> -	struct mmu_notifier_range range;
> -	unsigned long addr, i;
> -	bool notified = false;
> -
> -	for (i = 0, addr = start; i < npages; addr += PAGE_SIZE, i++) {
> -		struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
> -		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> -		struct address_space *mapping;
> -		int r;
> -
> -		if (!newpage) {
> -			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> -			continue;
> -		}
> -
> -		if (!page) {
> -			if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE))
> -				continue;
> -			if (!notified) {
> -				notified = true;
> -
> -				mmu_notifier_range_init_owner(&range,
> -					MMU_NOTIFY_MIGRATE, 0, migrate->vma,
> -					migrate->vma->vm_mm, addr, migrate->end,
> -					migrate->pgmap_owner);
> -				mmu_notifier_invalidate_range_start(&range);
> -			}
> -			migrate_vma_insert_page(migrate, addr, newpage,
> -						&migrate->src[i]);
> -			continue;
> -		}
> -
> -		mapping = page_mapping(page);
> -
> -		if (is_device_private_page(newpage)) {
> -			/*
> -			 * For now only support private anonymous when migrating
> -			 * to un-addressable device memory.
> -			 */
> -			if (mapping) {
> -				migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> -				continue;
> -			}
> -		} else if (is_zone_device_page(newpage)) {
> -			/*
> -			 * Other types of ZONE_DEVICE page are not supported.
> -			 */
> -			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> -			continue;
> -		}
> -
> -		r = migrate_page(mapping, newpage, page, MIGRATE_SYNC_NO_COPY);
> -		if (r != MIGRATEPAGE_SUCCESS)
> -			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> -	}
> -
> -	/*
> -	 * No need to double call mmu_notifier->invalidate_range() callback as
> -	 * the above ptep_clear_flush_notify() inside migrate_vma_insert_page()
> -	 * did already call it.
> -	 */
> -	if (notified)
> -		mmu_notifier_invalidate_range_only_end(&range);
> -}
> -EXPORT_SYMBOL(migrate_vma_pages);
> -
> -/**
> - * migrate_vma_finalize() - restore CPU page table entry
> - * @migrate: migrate struct containing all migration information
> - *
> - * This replaces the special migration pte entry with either a mapping to the
> - * new page if migration was successful for that page, or to the original page
> - * otherwise.
> - *
> - * This also unlocks the pages and puts them back on the lru, or drops the extra
> - * refcount, for device pages.
> - */
> -void migrate_vma_finalize(struct migrate_vma *migrate)
> -{
> -	const unsigned long npages = migrate->npages;
> -	unsigned long i;
> -
> -	for (i = 0; i < npages; i++) {
> -		struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
> -		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> -
> -		if (!page) {
> -			if (newpage) {
> -				unlock_page(newpage);
> -				put_page(newpage);
> -			}
> -			continue;
> -		}
> -
> -		if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE) || !newpage) {
> -			if (newpage) {
> -				unlock_page(newpage);
> -				put_page(newpage);
> -			}
> -			newpage = page;
> -		}
> -
> -		remove_migration_ptes(page, newpage, false);
> -		unlock_page(page);
> -
> -		if (is_zone_device_page(page))
> -			put_page(page);
> -		else
> -			putback_lru_page(page);
> -
> -		if (newpage != page) {
> -			unlock_page(newpage);
> -			if (is_zone_device_page(newpage))
> -				put_page(newpage);
> -			else
> -				putback_lru_page(newpage);
> -		}
> -	}
> -}
> -EXPORT_SYMBOL(migrate_vma_finalize);
> -#endif /* CONFIG_DEVICE_PRIVATE */
> -
>  /*
>   * node_demotion[] example:
>   *
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> new file mode 100644
> index 00000000000000..749e0bab8e4779
> --- /dev/null
> +++ b/mm/migrate_device.c
> @@ -0,0 +1,765 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Device Memory Migration functionality.
> + *
> + * Originally written by Jérôme Glisse.
> + */
> +#include <linux/export.h>
> +#include <linux/memremap.h>
> +#include <linux/migrate.h>
> +#include <linux/mm_inline.h>
> +#include <linux/mmu_notifier.h>
> +#include <linux/oom.h>
> +#include <linux/pagewalk.h>
> +#include <linux/rmap.h>
> +#include <linux/swapops.h>
> +#include "internal.h"
> +
> +static int migrate_vma_collect_skip(unsigned long start,
> +				    unsigned long end,
> +				    struct mm_walk *walk)
> +{
> +	struct migrate_vma *migrate = walk->private;
> +	unsigned long addr;
> +
> +	for (addr = start; addr < end; addr += PAGE_SIZE) {
> +		migrate->dst[migrate->npages] = 0;
> +		migrate->src[migrate->npages++] = 0;
> +	}
> +
> +	return 0;
> +}
> +
> +static int migrate_vma_collect_hole(unsigned long start,
> +				    unsigned long end,
> +				    __always_unused int depth,
> +				    struct mm_walk *walk)
> +{
> +	struct migrate_vma *migrate = walk->private;
> +	unsigned long addr;
> +
> +	/* Only allow populating anonymous memory. */
> +	if (!vma_is_anonymous(walk->vma))
> +		return migrate_vma_collect_skip(start, end, walk);
> +
> +	for (addr = start; addr < end; addr += PAGE_SIZE) {
> +		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
> +		migrate->dst[migrate->npages] = 0;
> +		migrate->npages++;
> +		migrate->cpages++;
> +	}
> +
> +	return 0;
> +}
> +
> +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;
> +	pte_t *ptep;
> +
> +again:
> +	if (pmd_none(*pmdp))
> +		return migrate_vma_collect_hole(start, end, -1, walk);
> +
> +	if (pmd_trans_huge(*pmdp)) {
> +		struct page *page;
> +
> +		ptl = pmd_lock(mm, pmdp);
> +		if (unlikely(!pmd_trans_huge(*pmdp))) {
> +			spin_unlock(ptl);
> +			goto again;
> +		}
> +
> +		page = pmd_page(*pmdp);
> +		if (is_huge_zero_page(page)) {
> +			spin_unlock(ptl);
> +			split_huge_pmd(vma, pmdp, addr);
> +			if (pmd_trans_unstable(pmdp))
> +				return migrate_vma_collect_skip(start, end,
> +								walk);
> +		} else {
> +			int ret;
> +
> +			get_page(page);
> +			spin_unlock(ptl);
> +			if (unlikely(!trylock_page(page)))
> +				return migrate_vma_collect_skip(start, end,
> +								walk);
> +			ret = split_huge_page(page);
> +			unlock_page(page);
> +			put_page(page);
> +			if (ret)
> +				return migrate_vma_collect_skip(start, end,
> +								walk);
> +			if (pmd_none(*pmdp))
> +				return migrate_vma_collect_hole(start, end, -1,
> +								walk);
> +		}
> +	}
> +
> +	if (unlikely(pmd_bad(*pmdp)))
> +		return migrate_vma_collect_skip(start, end, walk);
> +
> +	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> +	arch_enter_lazy_mmu_mode();
> +
> +	for (; addr < end; addr += PAGE_SIZE, ptep++) {
> +		unsigned long mpfn = 0, pfn;
> +		struct page *page;
> +		swp_entry_t entry;
> +		pte_t pte;
> +
> +		pte = *ptep;
> +
> +		if (pte_none(pte)) {
> +			if (vma_is_anonymous(vma)) {
> +				mpfn = MIGRATE_PFN_MIGRATE;
> +				migrate->cpages++;
> +			}
> +			goto next;
> +		}
> +
> +		if (!pte_present(pte)) {
> +			/*
> +			 * Only care about unaddressable device page special
> +			 * page table entry. Other special swap entries are not
> +			 * migratable, and we ignore regular swapped page.
> +			 */
> +			entry = pte_to_swp_entry(pte);
> +			if (!is_device_private_entry(entry))
> +				goto next;
> +
> +			page = pfn_swap_entry_to_page(entry);
> +			if (!(migrate->flags &
> +				MIGRATE_VMA_SELECT_DEVICE_PRIVATE) ||
> +			    page->pgmap->owner != migrate->pgmap_owner)
> +				goto next;
> +
> +			mpfn = migrate_pfn(page_to_pfn(page)) |
> +					MIGRATE_PFN_MIGRATE;
> +			if (is_writable_device_private_entry(entry))
> +				mpfn |= MIGRATE_PFN_WRITE;
> +		} else {
> +			if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM))
> +				goto next;
> +			pfn = pte_pfn(pte);
> +			if (is_zero_pfn(pfn)) {
> +				mpfn = MIGRATE_PFN_MIGRATE;
> +				migrate->cpages++;
> +				goto next;
> +			}
> +			page = vm_normal_page(migrate->vma, addr, pte);
> +			mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
> +			mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
> +		}
> +
> +		/* FIXME support THP */
> +		if (!page || !page->mapping || PageTransCompound(page)) {
> +			mpfn = 0;
> +			goto next;
> +		}
> +
> +		/*
> +		 * By getting a reference on the page we pin it and that blocks
> +		 * any kind of migration. Side effect is that it "freezes" the
> +		 * pte.
> +		 *
> +		 * We drop this reference after isolating the page from the lru
> +		 * for non device page (device page are not on the lru and thus
> +		 * can't be dropped from it).
> +		 */
> +		get_page(page);
> +
> +		/*
> +		 * Optimize for the common case where page is only mapped once
> +		 * in one process. If we can lock the page, then we can safely
> +		 * set up a special migration page table entry now.
> +		 */
> +		if (trylock_page(page)) {
> +			pte_t swp_pte;
> +
> +			migrate->cpages++;
> +			ptep_get_and_clear(mm, addr, ptep);
> +
> +			/* Setup special migration page table entry */
> +			if (mpfn & MIGRATE_PFN_WRITE)
> +				entry = make_writable_migration_entry(
> +							page_to_pfn(page));
> +			else
> +				entry = make_readable_migration_entry(
> +							page_to_pfn(page));
> +			swp_pte = swp_entry_to_pte(entry);
> +			if (pte_present(pte)) {
> +				if (pte_soft_dirty(pte))
> +					swp_pte = pte_swp_mksoft_dirty(swp_pte);
> +				if (pte_uffd_wp(pte))
> +					swp_pte = pte_swp_mkuffd_wp(swp_pte);
> +			} else {
> +				if (pte_swp_soft_dirty(pte))
> +					swp_pte = pte_swp_mksoft_dirty(swp_pte);
> +				if (pte_swp_uffd_wp(pte))
> +					swp_pte = pte_swp_mkuffd_wp(swp_pte);
> +			}
> +			set_pte_at(mm, addr, ptep, swp_pte);
> +
> +			/*
> +			 * This is like regular unmap: we remove the rmap and
> +			 * drop page refcount. Page won't be freed, as we took
> +			 * a reference just above.
> +			 */
> +			page_remove_rmap(page, false);
> +			put_page(page);
> +
> +			if (pte_present(pte))
> +				unmapped++;
> +		} else {
> +			put_page(page);
> +			mpfn = 0;
> +		}
> +
> +next:
> +		migrate->dst[migrate->npages] = 0;
> +		migrate->src[migrate->npages++] = mpfn;
> +	}
> +	arch_leave_lazy_mmu_mode();
> +	pte_unmap_unlock(ptep - 1, ptl);
> +
> +	/* Only flush the TLB if we actually modified any entries */
> +	if (unmapped)
> +		flush_tlb_range(walk->vma, start, end);
> +
> +	return 0;
> +}
> +
> +static const struct mm_walk_ops migrate_vma_walk_ops = {
> +	.pmd_entry		= migrate_vma_collect_pmd,
> +	.pte_hole		= migrate_vma_collect_hole,
> +};
> +
> +/*
> + * migrate_vma_collect() - collect pages over a range of virtual addresses
> + * @migrate: migrate struct containing all migration information
> + *
> + * This will walk the CPU page table. For each virtual address backed by a
> + * valid page, it updates the src array and takes a reference on the page, in
> + * order to pin the page until we lock it and unmap it.
> + */
> +static void migrate_vma_collect(struct migrate_vma *migrate)
> +{
> +	struct mmu_notifier_range range;
> +
> +	/*
> +	 * Note that the pgmap_owner is passed to the mmu notifier callback so
> +	 * that the registered device driver can skip invalidating device
> +	 * private page mappings that won't be migrated.
> +	 */
> +	mmu_notifier_range_init_owner(&range, MMU_NOTIFY_MIGRATE, 0,
> +		migrate->vma, migrate->vma->vm_mm, migrate->start, migrate->end,
> +		migrate->pgmap_owner);
> +	mmu_notifier_invalidate_range_start(&range);
> +
> +	walk_page_range(migrate->vma->vm_mm, migrate->start, migrate->end,
> +			&migrate_vma_walk_ops, migrate);
> +
> +	mmu_notifier_invalidate_range_end(&range);
> +	migrate->end = migrate->start + (migrate->npages << PAGE_SHIFT);
> +}
> +
> +/*
> + * migrate_vma_check_page() - check if page is pinned or not
> + * @page: struct page to check
> + *
> + * Pinned pages cannot be migrated. This is the same test as in
> + * folio_migrate_mapping(), except that here we allow migration of a
> + * ZONE_DEVICE page.
> + */
> +static bool migrate_vma_check_page(struct page *page)
> +{
> +	/*
> +	 * One extra ref because caller holds an extra reference, either from
> +	 * isolate_lru_page() for a regular page, or migrate_vma_collect() for
> +	 * a device page.
> +	 */
> +	int extra = 1;
> +
> +	/*
> +	 * 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 (PageCompound(page))
> +		return false;
> +
> +	/* Page from ZONE_DEVICE have one extra reference */
> +	if (is_zone_device_page(page))
> +		extra++;
> +
> +	/* For file back page */
> +	if (page_mapping(page))
> +		extra += 1 + page_has_private(page);
> +
> +	if ((page_count(page) - extra) > page_mapcount(page))
> +		return false;
> +
> +	return true;
> +}
> +
> +/*
> + * migrate_vma_unmap() - replace page mapping with special migration pte entry
> + * @migrate: migrate struct containing all migration information
> + *
> + * Isolate pages from the LRU and replace mappings (CPU page table pte) with a
> + * special migration pte entry and check if it has been pinned. Pinned pages are
> + * restored because we cannot migrate them.
> + *
> + * This is the last step before we call the device driver callback to allocate
> + * destination memory and copy contents of original page over to new page.
> + */
> +static void migrate_vma_unmap(struct migrate_vma *migrate)
> +{
> +	const unsigned long npages = migrate->npages;
> +	unsigned long i, restore = 0;
> +	bool allow_drain = true;
> +
> +	lru_add_drain();
> +
> +	for (i = 0; i < npages; i++) {
> +		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> +
> +		if (!page)
> +			continue;
> +
> +		/* ZONE_DEVICE pages are not on LRU */
> +		if (!is_zone_device_page(page)) {
> +			if (!PageLRU(page) && allow_drain) {
> +				/* Drain CPU's pagevec */
> +				lru_add_drain_all();
> +				allow_drain = false;
> +			}
> +
> +			if (isolate_lru_page(page)) {
> +				migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> +				migrate->cpages--;
> +				restore++;
> +				continue;
> +			}
> +
> +			/* Drop the reference we took in collect */
> +			put_page(page);
> +		}
> +
> +		if (page_mapped(page))
> +			try_to_migrate(page, 0);
> +
> +		if (page_mapped(page) || !migrate_vma_check_page(page)) {
> +			if (!is_zone_device_page(page)) {
> +				get_page(page);
> +				putback_lru_page(page);
> +			}
> +
> +			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> +			migrate->cpages--;
> +			restore++;
> +			continue;
> +		}
> +	}
> +
> +	for (i = 0; i < npages && restore; i++) {
> +		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> +
> +		if (!page || (migrate->src[i] & MIGRATE_PFN_MIGRATE))
> +			continue;
> +
> +		remove_migration_ptes(page, page, false);
> +
> +		migrate->src[i] = 0;
> +		unlock_page(page);
> +		put_page(page);
> +		restore--;
> +	}
> +}
> +
> +/**
> + * migrate_vma_setup() - prepare to migrate a range of memory
> + * @args: contains the vma, start, and pfns arrays for the migration
> + *
> + * Returns: negative errno on failures, 0 when 0 or more pages were migrated
> + * without an error.
> + *
> + * Prepare to migrate a range of memory virtual address range by collecting all
> + * the pages backing each virtual address in the range, saving them inside the
> + * src array.  Then lock those pages and unmap them. Once the pages are locked
> + * and unmapped, check whether each page is pinned or not.  Pages that aren't
> + * pinned have the MIGRATE_PFN_MIGRATE flag set (by this function) in the
> + * corresponding src array entry.  Then restores any pages that are pinned, by
> + * remapping and unlocking those pages.
> + *
> + * The caller should then allocate destination memory and copy source memory to
> + * it for all those entries (ie with MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE
> + * flag set).  Once these are allocated and copied, the caller must update each
> + * corresponding entry in the dst array with the pfn value of the destination
> + * page and with MIGRATE_PFN_VALID. Destination pages must be locked via
> + * lock_page().
> + *
> + * Note that the caller does not have to migrate all the pages that are marked
> + * with MIGRATE_PFN_MIGRATE flag in src array unless this is a migration from
> + * device memory to system memory.  If the caller cannot migrate a device page
> + * back to system memory, then it must return VM_FAULT_SIGBUS, which has severe
> + * consequences for the userspace process, so it must be avoided if at all
> + * possible.
> + *
> + * For empty entries inside CPU page table (pte_none() or pmd_none() is true) we
> + * do set MIGRATE_PFN_MIGRATE flag inside the corresponding source array thus
> + * allowing the caller to allocate device memory for those unbacked virtual
> + * addresses.  For this the caller simply has to allocate device memory and
> + * properly set the destination entry like for regular migration.  Note that
> + * this can still fail, and thus inside the device driver you must check if the
> + * migration was successful for those entries after calling migrate_vma_pages(),
> + * just like for regular migration.
> + *
> + * After that, the callers must call migrate_vma_pages() to go over each entry
> + * in the src array that has the MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag
> + * set. If the corresponding entry in dst array has MIGRATE_PFN_VALID flag set,
> + * then migrate_vma_pages() to migrate struct page information from the source
> + * struct page to the destination struct page.  If it fails to migrate the
> + * struct page information, then it clears the MIGRATE_PFN_MIGRATE flag in the
> + * src array.
> + *
> + * At this point all successfully migrated pages have an entry in the src
> + * array with MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag set and the dst
> + * array entry with MIGRATE_PFN_VALID flag set.
> + *
> + * Once migrate_vma_pages() returns the caller may inspect which pages were
> + * successfully migrated, and which were not.  Successfully migrated pages will
> + * have the MIGRATE_PFN_MIGRATE flag set for their src array entry.
> + *
> + * It is safe to update device page table after migrate_vma_pages() because
> + * both destination and source page are still locked, and the mmap_lock is held
> + * in read mode (hence no one can unmap the range being migrated).
> + *
> + * Once the caller is done cleaning up things and updating its page table (if it
> + * chose to do so, this is not an obligation) it finally calls
> + * migrate_vma_finalize() to update the CPU page table to point to new pages
> + * for successfully migrated pages or otherwise restore the CPU page table to
> + * point to the original source pages.
> + */
> +int migrate_vma_setup(struct migrate_vma *args)
> +{
> +	long nr_pages = (args->end - args->start) >> PAGE_SHIFT;
> +
> +	args->start &= PAGE_MASK;
> +	args->end &= PAGE_MASK;
> +	if (!args->vma || is_vm_hugetlb_page(args->vma) ||
> +	    (args->vma->vm_flags & VM_SPECIAL) || vma_is_dax(args->vma))
> +		return -EINVAL;
> +	if (nr_pages <= 0)
> +		return -EINVAL;
> +	if (args->start < args->vma->vm_start ||
> +	    args->start >= args->vma->vm_end)
> +		return -EINVAL;
> +	if (args->end <= args->vma->vm_start || args->end > args->vma->vm_end)
> +		return -EINVAL;
> +	if (!args->src || !args->dst)
> +		return -EINVAL;
> +
> +	memset(args->src, 0, sizeof(*args->src) * nr_pages);
> +	args->cpages = 0;
> +	args->npages = 0;
> +
> +	migrate_vma_collect(args);
> +
> +	if (args->cpages)
> +		migrate_vma_unmap(args);
> +
> +	/*
> +	 * At this point pages are locked and unmapped, and thus they have
> +	 * stable content and can safely be copied to destination memory that
> +	 * is allocated by the drivers.
> +	 */
> +	return 0;
> +
> +}
> +EXPORT_SYMBOL(migrate_vma_setup);
> +
> +/*
> + * This code closely matches the code in:
> + *   __handle_mm_fault()
> + *     handle_pte_fault()
> + *       do_anonymous_page()
> + * to map in an anonymous zero page but the struct page will be a ZONE_DEVICE
> + * private page.
> + */
> +static void migrate_vma_insert_page(struct migrate_vma *migrate,
> +				    unsigned long addr,
> +				    struct page *page,
> +				    unsigned long *src)
> +{
> +	struct vm_area_struct *vma = migrate->vma;
> +	struct mm_struct *mm = vma->vm_mm;
> +	bool flush = false;
> +	spinlock_t *ptl;
> +	pte_t entry;
> +	pgd_t *pgdp;
> +	p4d_t *p4dp;
> +	pud_t *pudp;
> +	pmd_t *pmdp;
> +	pte_t *ptep;
> +
> +	/* Only allow populating anonymous memory */
> +	if (!vma_is_anonymous(vma))
> +		goto abort;
> +
> +	pgdp = pgd_offset(mm, addr);
> +	p4dp = p4d_alloc(mm, pgdp, addr);
> +	if (!p4dp)
> +		goto abort;
> +	pudp = pud_alloc(mm, p4dp, addr);
> +	if (!pudp)
> +		goto abort;
> +	pmdp = pmd_alloc(mm, pudp, addr);
> +	if (!pmdp)
> +		goto abort;
> +
> +	if (pmd_trans_huge(*pmdp) || pmd_devmap(*pmdp))
> +		goto abort;
> +
> +	/*
> +	 * Use pte_alloc() instead of pte_alloc_map().  We can't run
> +	 * pte_offset_map() on pmds where a huge pmd might be created
> +	 * from a different thread.
> +	 *
> +	 * pte_alloc_map() is safe to use under mmap_write_lock(mm) or when
> +	 * parallel threads are excluded by other means.
> +	 *
> +	 * Here we only have mmap_read_lock(mm).
> +	 */
> +	if (pte_alloc(mm, pmdp))
> +		goto abort;
> +
> +	/* See the comment in pte_alloc_one_map() */
> +	if (unlikely(pmd_trans_unstable(pmdp)))
> +		goto abort;
> +
> +	if (unlikely(anon_vma_prepare(vma)))
> +		goto abort;
> +	if (mem_cgroup_charge(page_folio(page), vma->vm_mm, GFP_KERNEL))
> +		goto abort;
> +
> +	/*
> +	 * The memory barrier inside __SetPageUptodate makes sure that
> +	 * preceding stores to the page contents become visible before
> +	 * the set_pte_at() write.
> +	 */
> +	__SetPageUptodate(page);
> +
> +	if (is_device_private_page(page)) {
> +		swp_entry_t swp_entry;
> +
> +		if (vma->vm_flags & VM_WRITE)
> +			swp_entry = make_writable_device_private_entry(
> +						page_to_pfn(page));
> +		else
> +			swp_entry = make_readable_device_private_entry(
> +						page_to_pfn(page));
> +		entry = swp_entry_to_pte(swp_entry);
> +	} else {
> +		/*
> +		 * For now we only support migrating to un-addressable device
> +		 * memory.
> +		 */
> +		if (is_zone_device_page(page)) {
> +			pr_warn_once("Unsupported ZONE_DEVICE page type.\n");
> +			goto abort;
> +		}
> +		entry = mk_pte(page, vma->vm_page_prot);
> +		if (vma->vm_flags & VM_WRITE)
> +			entry = pte_mkwrite(pte_mkdirty(entry));
> +	}
> +
> +	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> +
> +	if (check_stable_address_space(mm))
> +		goto unlock_abort;
> +
> +	if (pte_present(*ptep)) {
> +		unsigned long pfn = pte_pfn(*ptep);
> +
> +		if (!is_zero_pfn(pfn))
> +			goto unlock_abort;
> +		flush = true;
> +	} else if (!pte_none(*ptep))
> +		goto unlock_abort;
> +
> +	/*
> +	 * Check for userfaultfd but do not deliver the fault. Instead,
> +	 * just back off.
> +	 */
> +	if (userfaultfd_missing(vma))
> +		goto unlock_abort;
> +
> +	inc_mm_counter(mm, MM_ANONPAGES);
> +	page_add_new_anon_rmap(page, vma, addr, false);
> +	if (!is_zone_device_page(page))
> +		lru_cache_add_inactive_or_unevictable(page, vma);
> +	get_page(page);
> +
> +	if (flush) {
> +		flush_cache_page(vma, addr, pte_pfn(*ptep));
> +		ptep_clear_flush_notify(vma, addr, ptep);
> +		set_pte_at_notify(mm, addr, ptep, entry);
> +		update_mmu_cache(vma, addr, ptep);
> +	} else {
> +		/* No need to invalidate - it was non-present before */
> +		set_pte_at(mm, addr, ptep, entry);
> +		update_mmu_cache(vma, addr, ptep);
> +	}
> +
> +	pte_unmap_unlock(ptep, ptl);
> +	*src = MIGRATE_PFN_MIGRATE;
> +	return;
> +
> +unlock_abort:
> +	pte_unmap_unlock(ptep, ptl);
> +abort:
> +	*src &= ~MIGRATE_PFN_MIGRATE;
> +}
> +
> +/**
> + * migrate_vma_pages() - migrate meta-data from src page to dst page
> + * @migrate: migrate struct containing all migration information
> + *
> + * This migrates struct page meta-data from source struct page to destination
> + * struct page. This effectively finishes the migration from source page to the
> + * destination page.
> + */
> +void migrate_vma_pages(struct migrate_vma *migrate)
> +{
> +	const unsigned long npages = migrate->npages;
> +	const unsigned long start = migrate->start;
> +	struct mmu_notifier_range range;
> +	unsigned long addr, i;
> +	bool notified = false;
> +
> +	for (i = 0, addr = start; i < npages; addr += PAGE_SIZE, i++) {
> +		struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
> +		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> +		struct address_space *mapping;
> +		int r;
> +
> +		if (!newpage) {
> +			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> +			continue;
> +		}
> +
> +		if (!page) {
> +			if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE))
> +				continue;
> +			if (!notified) {
> +				notified = true;
> +
> +				mmu_notifier_range_init_owner(&range,
> +					MMU_NOTIFY_MIGRATE, 0, migrate->vma,
> +					migrate->vma->vm_mm, addr, migrate->end,
> +					migrate->pgmap_owner);
> +				mmu_notifier_invalidate_range_start(&range);
> +			}
> +			migrate_vma_insert_page(migrate, addr, newpage,
> +						&migrate->src[i]);
> +			continue;
> +		}
> +
> +		mapping = page_mapping(page);
> +
> +		if (is_device_private_page(newpage)) {
> +			/*
> +			 * For now only support private anonymous when migrating
> +			 * to un-addressable device memory.
> +			 */
> +			if (mapping) {
> +				migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> +				continue;
> +			}
> +		} else if (is_zone_device_page(newpage)) {
> +			/*
> +			 * Other types of ZONE_DEVICE page are not supported.
> +			 */
> +			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> +			continue;
> +		}
> +
> +		r = migrate_page(mapping, newpage, page, MIGRATE_SYNC_NO_COPY);
> +		if (r != MIGRATEPAGE_SUCCESS)
> +			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
> +	}
> +
> +	/*
> +	 * No need to double call mmu_notifier->invalidate_range() callback as
> +	 * the above ptep_clear_flush_notify() inside migrate_vma_insert_page()
> +	 * did already call it.
> +	 */
> +	if (notified)
> +		mmu_notifier_invalidate_range_only_end(&range);
> +}
> +EXPORT_SYMBOL(migrate_vma_pages);
> +
> +/**
> + * migrate_vma_finalize() - restore CPU page table entry
> + * @migrate: migrate struct containing all migration information
> + *
> + * This replaces the special migration pte entry with either a mapping to the
> + * new page if migration was successful for that page, or to the original page
> + * otherwise.
> + *
> + * This also unlocks the pages and puts them back on the lru, or drops the extra
> + * refcount, for device pages.
> + */
> +void migrate_vma_finalize(struct migrate_vma *migrate)
> +{
> +	const unsigned long npages = migrate->npages;
> +	unsigned long i;
> +
> +	for (i = 0; i < npages; i++) {
> +		struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
> +		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> +
> +		if (!page) {
> +			if (newpage) {
> +				unlock_page(newpage);
> +				put_page(newpage);
> +			}
> +			continue;
> +		}
> +
> +		if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE) || !newpage) {
> +			if (newpage) {
> +				unlock_page(newpage);
> +				put_page(newpage);
> +			}
> +			newpage = page;
> +		}
> +
> +		remove_migration_ptes(page, newpage, false);
> +		unlock_page(page);
> +
> +		if (is_zone_device_page(page))
> +			put_page(page);
> +		else
> +			putback_lru_page(page);
> +
> +		if (newpage != page) {
> +			unlock_page(newpage);
> +			if (is_zone_device_page(newpage))
> +				put_page(newpage);
> +			else
> +				putback_lru_page(newpage);
> +		}
> +	}
> +}
> +EXPORT_SYMBOL(migrate_vma_finalize);
>