Merge tag 'sched_ext-for-6.12' of git://git.kernel.org/pub/scm/linux/kernel/git/tj...

[thirdparty/linux.git] / mm / migrate_device.c

// 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.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 <asm/tlbflush.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 folio *folio;

                ptl = pmd_lock(mm, pmdp);
                if (unlikely(!pmd_trans_huge(*pmdp))) {
                        spin_unlock(ptl);
                        goto again;
                }

                folio = pmd_folio(*pmdp);
                if (is_huge_zero_folio(folio)) {
                        spin_unlock(ptl);
                        split_huge_pmd(vma, pmdp, addr);
                } else {
                        int ret;

                        folio_get(folio);
                        spin_unlock(ptl);
                        if (unlikely(!folio_trylock(folio)))
                                return migrate_vma_collect_skip(start, end,
                                                                walk);
                        ret = split_folio(folio);
                        folio_unlock(folio);
                        folio_put(folio);
                        if (ret)
                                return migrate_vma_collect_skip(start, end,
                                                                walk);
                }
        }

        ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
        if (!ptep)
                goto again;
        arch_enter_lazy_mmu_mode();

        for (; addr < end; addr += PAGE_SIZE, ptep++) {
                unsigned long mpfn = 0, pfn;
                struct folio *folio;
                struct page *page;
                swp_entry_t entry;
                pte_t pte;

                pte = ptep_get(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 {
                        pfn = pte_pfn(pte);
                        if (is_zero_pfn(pfn) &&
                            (migrate->flags & MIGRATE_VMA_SELECT_SYSTEM)) {
                                mpfn = MIGRATE_PFN_MIGRATE;
                                migrate->cpages++;
                                goto next;
                        }
                        page = vm_normal_page(migrate->vma, addr, pte);
                        if (page && !is_zone_device_page(page) &&
                            !(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM))
                                goto next;
                        else if (page && is_device_coherent_page(page) &&
                            (!(migrate->flags & MIGRATE_VMA_SELECT_DEVICE_COHERENT) ||
                             page->pgmap->owner != migrate->pgmap_owner))
                                goto next;
                        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 folio 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 folio from the lru
                 * for non device folio (device folio are not on the lru and thus
                 * can't be dropped from it).
                 */
                folio = page_folio(page);
                folio_get(folio);

                /*
                 * We rely on folio_trylock() to avoid deadlock between
                 * concurrent migrations where each is waiting on the others
                 * folio lock. If we can't immediately lock the folio we fail this
                 * migration as it is only best effort anyway.
                 *
                 * If we can lock the folio it's safe to set up a migration entry
                 * now. In the common case where the folio is mapped once in a
                 * single process setting up the migration entry now is an
                 * optimisation to avoid walking the rmap later with
                 * try_to_migrate().
                 */
                if (folio_trylock(folio)) {
                        bool anon_exclusive;
                        pte_t swp_pte;

                        flush_cache_page(vma, addr, pte_pfn(pte));
                        anon_exclusive = folio_test_anon(folio) &&
                                          PageAnonExclusive(page);
                        if (anon_exclusive) {
                                pte = ptep_clear_flush(vma, addr, ptep);

                                if (folio_try_share_anon_rmap_pte(folio, page)) {
                                        set_pte_at(mm, addr, ptep, pte);
                                        folio_unlock(folio);
                                        folio_put(folio);
                                        mpfn = 0;
                                        goto next;
                                }
                        } else {
                                pte = ptep_get_and_clear(mm, addr, ptep);
                        }

                        migrate->cpages++;

                        /* Set the dirty flag on the folio now the pte is gone. */
                        if (pte_dirty(pte))
                                folio_mark_dirty(folio);

                        /* Setup special migration page table entry */
                        if (mpfn & MIGRATE_PFN_WRITE)
                                entry = make_writable_migration_entry(
                                                        page_to_pfn(page));
                        else if (anon_exclusive)
                                entry = make_readable_exclusive_migration_entry(
                                                        page_to_pfn(page));
                        else
                                entry = make_readable_migration_entry(
                                                        page_to_pfn(page));
                        if (pte_present(pte)) {
                                if (pte_young(pte))
                                        entry = make_migration_entry_young(entry);
                                if (pte_dirty(pte))
                                        entry = make_migration_entry_dirty(entry);
                        }
                        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 the folio refcount. The folio won't be freed, as
                         * we took a reference just above.
                         */
                        folio_remove_rmap_pte(folio, page, vma);
                        folio_put(folio);

                        if (pte_present(pte))
                                unmapped++;
                } else {
                        folio_put(folio);
                        mpfn = 0;
                }

next:
                migrate->dst[migrate->npages] = 0;
                migrate->src[migrate->npages++] = mpfn;
        }

        /* Only flush the TLB if we actually modified any entries */
        if (unmapped)
                flush_tlb_range(walk->vma, start, end);

        arch_leave_lazy_mmu_mode();
        pte_unmap_unlock(ptep - 1, ptl);

        return 0;
}

static const struct mm_walk_ops migrate_vma_walk_ops = {
        .pmd_entry              = migrate_vma_collect_pmd,
        .pte_hole               = migrate_vma_collect_hole,
        .walk_lock              = PGWALK_RDLOCK,
};

/*
 * 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->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, struct page *fault_page)
{
        struct folio *folio = page_folio(page);

        /*
         * One extra ref because caller holds an extra reference, either from
         * folio_isolate_lru() for a regular folio, or migrate_vma_collect() for
         * a device folio.
         */
        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;

        /* Page from ZONE_DEVICE have one extra reference */
        if (folio_is_zone_device(folio))
                extra++;

        /* For file back page */
        if (folio_mapping(folio))
                extra += 1 + folio_has_private(folio);

        if ((folio_ref_count(folio) - extra) > folio_mapcount(folio))
                return false;

        return true;
}

/*
 * Unmaps pages for migration. Returns number of source pfns marked as
 * migrating.
 */
static unsigned long migrate_device_unmap(unsigned long *src_pfns,
                                          unsigned long npages,
                                          struct page *fault_page)
{
        unsigned long i, restore = 0;
        bool allow_drain = true;
        unsigned long unmapped = 0;

        lru_add_drain();

        for (i = 0; i < npages; i++) {
                struct page *page = migrate_pfn_to_page(src_pfns[i]);
                struct folio *folio;

                if (!page) {
                        if (src_pfns[i] & MIGRATE_PFN_MIGRATE)
                                unmapped++;
                        continue;
                }

                folio = page_folio(page);
                /* ZONE_DEVICE folios are not on LRU */
                if (!folio_is_zone_device(folio)) {
                        if (!folio_test_lru(folio) && allow_drain) {
                                /* Drain CPU's lru cache */
                                lru_add_drain_all();
                                allow_drain = false;
                        }

                        if (!folio_isolate_lru(folio)) {
                                src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
                                restore++;
                                continue;
                        }

                        /* Drop the reference we took in collect */
                        folio_put(folio);
                }

                if (folio_mapped(folio))
                        try_to_migrate(folio, 0);

                if (folio_mapped(folio) ||
                    !migrate_vma_check_page(page, fault_page)) {
                        if (!folio_is_zone_device(folio)) {
                                folio_get(folio);
                                folio_putback_lru(folio);
                        }

                        src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
                        restore++;
                        continue;
                }

                unmapped++;
        }

        for (i = 0; i < npages && restore; i++) {
                struct page *page = migrate_pfn_to_page(src_pfns[i]);
                struct folio *folio;

                if (!page || (src_pfns[i] & MIGRATE_PFN_MIGRATE))
                        continue;

                folio = page_folio(page);
                remove_migration_ptes(folio, folio, 0);

                src_pfns[i] = 0;
                folio_unlock(folio);
                folio_put(folio);
                restore--;
        }

        return unmapped;
}

/*
 * 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)
{
        migrate->cpages = migrate_device_unmap(migrate->src, migrate->npages,
                                        migrate->fault_page);
}

/**
 * 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;
        if (args->fault_page && !is_device_private_page(args->fault_page))
                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 or coherent page.
 */
static void migrate_vma_insert_page(struct migrate_vma *migrate,
                                    unsigned long addr,
                                    struct page *page,
                                    unsigned long *src)
{
        struct folio *folio = page_folio(page);
        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;
        pte_t orig_pte;

        /* 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;
        if (pte_alloc(mm, pmdp))
                goto abort;
        if (unlikely(anon_vma_prepare(vma)))
                goto abort;
        if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL))
                goto abort;

        /*
         * The memory barrier inside __folio_mark_uptodate makes sure that
         * preceding stores to the folio contents become visible before
         * the set_pte_at() write.
         */
        __folio_mark_uptodate(folio);

        if (folio_is_device_private(folio)) {
                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 {
                if (folio_is_zone_device(folio) &&
                    !folio_is_device_coherent(folio)) {
                        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), vma);
        }

        ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
        if (!ptep)
                goto abort;
        orig_pte = ptep_get(ptep);

        if (check_stable_address_space(mm))
                goto unlock_abort;

        if (pte_present(orig_pte)) {
                unsigned long pfn = pte_pfn(orig_pte);

                if (!is_zero_pfn(pfn))
                        goto unlock_abort;
                flush = true;
        } else if (!pte_none(orig_pte))
                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);
        folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);
        if (!folio_is_zone_device(folio))
                folio_add_lru_vma(folio, vma);
        folio_get(folio);

        if (flush) {
                flush_cache_page(vma, addr, pte_pfn(orig_pte));
                ptep_clear_flush(vma, addr, ptep);
        }
        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;
}

static void __migrate_device_pages(unsigned long *src_pfns,
                                unsigned long *dst_pfns, unsigned long npages,
                                struct migrate_vma *migrate)
{
        struct mmu_notifier_range range;
        unsigned long i;
        bool notified = false;

        for (i = 0; i < npages; i++) {
                struct page *newpage = migrate_pfn_to_page(dst_pfns[i]);
                struct page *page = migrate_pfn_to_page(src_pfns[i]);
                struct address_space *mapping;
                struct folio *newfolio, *folio;
                int r, extra_cnt = 0;

                if (!newpage) {
                        src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
                        continue;
                }

                if (!page) {
                        unsigned long addr;

                        if (!(src_pfns[i] & MIGRATE_PFN_MIGRATE))
                                continue;

                        /*
                         * The only time there is no vma is when called from
                         * migrate_device_coherent_folio(). However this isn't
                         * called if the page could not be unmapped.
                         */
                        VM_BUG_ON(!migrate);
                        addr = migrate->start + i*PAGE_SIZE;
                        if (!notified) {
                                notified = true;

                                mmu_notifier_range_init_owner(&range,
                                        MMU_NOTIFY_MIGRATE, 0,
                                        migrate->vma->vm_mm, addr, migrate->end,
                                        migrate->pgmap_owner);
                                mmu_notifier_invalidate_range_start(&range);
                        }
                        migrate_vma_insert_page(migrate, addr, newpage,
                                                &src_pfns[i]);
                        continue;
                }

                newfolio = page_folio(newpage);
                folio = page_folio(page);
                mapping = folio_mapping(folio);

                if (folio_is_device_private(newfolio) ||
                    folio_is_device_coherent(newfolio)) {
                        if (mapping) {
                                /*
                                 * For now only support anonymous memory migrating to
                                 * device private or coherent memory.
                                 *
                                 * Try to get rid of swap cache if possible.
                                 */
                                if (!folio_test_anon(folio) ||
                                    !folio_free_swap(folio)) {
                                        src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
                                        continue;
                                }
                        }
                } else if (folio_is_zone_device(newfolio)) {
                        /*
                         * Other types of ZONE_DEVICE page are not supported.
                         */
                        src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
                        continue;
                }

                BUG_ON(folio_test_writeback(folio));

                if (migrate && migrate->fault_page == page)
                        extra_cnt = 1;
                r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt);
                if (r != MIGRATEPAGE_SUCCESS)
                        src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
                else
                        folio_migrate_flags(newfolio, folio);
        }

        if (notified)
                mmu_notifier_invalidate_range_end(&range);
}

/**
 * migrate_device_pages() - migrate meta-data from src page to dst page
 * @src_pfns: src_pfns returned from migrate_device_range()
 * @dst_pfns: array of pfns allocated by the driver to migrate memory to
 * @npages: number of pages in the range
 *
 * Equivalent to migrate_vma_pages(). This is called to migrate struct page
 * meta-data from source struct page to destination.
 */
void migrate_device_pages(unsigned long *src_pfns, unsigned long *dst_pfns,
                        unsigned long npages)
{
        __migrate_device_pages(src_pfns, dst_pfns, npages, NULL);
}
EXPORT_SYMBOL(migrate_device_pages);

/**
 * 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)
{
        __migrate_device_pages(migrate->src, migrate->dst, migrate->npages, migrate);
}
EXPORT_SYMBOL(migrate_vma_pages);

/*
 * migrate_device_finalize() - complete page migration
 * @src_pfns: src_pfns returned from migrate_device_range()
 * @dst_pfns: array of pfns allocated by the driver to migrate memory to
 * @npages: number of pages in the range
 *
 * Completes migration of the page by removing special migration entries.
 * Drivers must ensure copying of page data is complete and visible to the CPU
 * before calling this.
 */
void migrate_device_finalize(unsigned long *src_pfns,
                        unsigned long *dst_pfns, unsigned long npages)
{
        unsigned long i;

        for (i = 0; i < npages; i++) {
                struct folio *dst = NULL, *src = NULL;
                struct page *newpage = migrate_pfn_to_page(dst_pfns[i]);
                struct page *page = migrate_pfn_to_page(src_pfns[i]);

                if (newpage)
                        dst = page_folio(newpage);

                if (!page) {
                        if (dst) {
                                folio_unlock(dst);
                                folio_put(dst);
                        }
                        continue;
                }

                src = page_folio(page);

                if (!(src_pfns[i] & MIGRATE_PFN_MIGRATE) || !dst) {
                        if (dst) {
                                folio_unlock(dst);
                                folio_put(dst);
                        }
                        dst = src;
                }

                remove_migration_ptes(src, dst, 0);
                folio_unlock(src);

                if (folio_is_zone_device(src))
                        folio_put(src);
                else
                        folio_putback_lru(src);

                if (dst != src) {
                        folio_unlock(dst);
                        if (folio_is_zone_device(dst))
                                folio_put(dst);
                        else
                                folio_putback_lru(dst);
                }
        }
}
EXPORT_SYMBOL(migrate_device_finalize);

/**
 * 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)
{
        migrate_device_finalize(migrate->src, migrate->dst, migrate->npages);
}
EXPORT_SYMBOL(migrate_vma_finalize);

/**
 * migrate_device_range() - migrate device private pfns to normal memory.
 * @src_pfns: array large enough to hold migrating source device private pfns.
 * @start: starting pfn in the range to migrate.
 * @npages: number of pages to migrate.
 *
 * migrate_vma_setup() is similar in concept to migrate_vma_setup() except that
 * instead of looking up pages based on virtual address mappings a range of
 * device pfns that should be migrated to system memory is used instead.
 *
 * This is useful when a driver needs to free device memory but doesn't know the
 * virtual mappings of every page that may be in device memory. For example this
 * is often the case when a driver is being unloaded or unbound from a device.
 *
 * Like migrate_vma_setup() this function will take a reference and lock any
 * migrating pages that aren't free before unmapping them. Drivers may then
 * allocate destination pages and start copying data from the device to CPU
 * memory before calling migrate_device_pages().
 */
int migrate_device_range(unsigned long *src_pfns, unsigned long start,
                        unsigned long npages)
{
        unsigned long i, pfn;

        for (pfn = start, i = 0; i < npages; pfn++, i++) {
                struct folio *folio;

                folio = folio_get_nontail_page(pfn_to_page(pfn));
                if (!folio) {
                        src_pfns[i] = 0;
                        continue;
                }

                if (!folio_trylock(folio)) {
                        src_pfns[i] = 0;
                        folio_put(folio);
                        continue;
                }

                src_pfns[i] = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
        }

        migrate_device_unmap(src_pfns, npages, NULL);

        return 0;
}
EXPORT_SYMBOL(migrate_device_range);

/*
 * Migrate a device coherent folio back to normal memory. The caller should have
 * a reference on folio which will be copied to the new folio if migration is
 * successful or dropped on failure.
 */
int migrate_device_coherent_folio(struct folio *folio)
{
        unsigned long src_pfn, dst_pfn = 0;
        struct folio *dfolio;

        WARN_ON_ONCE(folio_test_large(folio));

        folio_lock(folio);
        src_pfn = migrate_pfn(folio_pfn(folio)) | MIGRATE_PFN_MIGRATE;

        /*
         * We don't have a VMA and don't need to walk the page tables to find
         * the source folio. So call migrate_vma_unmap() directly to unmap the
         * folio as migrate_vma_setup() will fail if args.vma == NULL.
         */
        migrate_device_unmap(&src_pfn, 1, NULL);
        if (!(src_pfn & MIGRATE_PFN_MIGRATE))
                return -EBUSY;

        dfolio = folio_alloc(GFP_USER | __GFP_NOWARN, 0);
        if (dfolio) {
                folio_lock(dfolio);
                dst_pfn = migrate_pfn(folio_pfn(dfolio));
        }

        migrate_device_pages(&src_pfn, &dst_pfn, 1);
        if (src_pfn & MIGRATE_PFN_MIGRATE)
                folio_copy(dfolio, folio);
        migrate_device_finalize(&src_pfn, &dst_pfn, 1);

        if (src_pfn & MIGRATE_PFN_MIGRATE)
                return 0;
        return -EBUSY;
}