// SPDX-License-Identifier: GPL-2.0-only
/*
 *  mm/userfaultfd.c
 *
 *  Copyright (C) 2015  Red Hat, Inc.
 */

#include <linux/mm.h>
#include <linux/sched/signal.h>
#include <linux/pagemap.h>
#include <linux/rmap.h>
#include <linux/swap.h>
#include <linux/leafops.h>
#include <linux/userfaultfd_k.h>
#include <linux/mmu_notifier.h>
#include <linux/hugetlb.h>
#include <linux/shmem_fs.h>
#include <asm/tlbflush.h>
#include <asm/tlb.h>
#include "internal.h"
#include "swap.h"

static __always_inline
bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end)
{
        /* Make sure that the dst range is fully within dst_vma. */
        if (dst_end > dst_vma->vm_end)
                return false;

        /*
         * Check the vma is registered in uffd, this is required to
         * enforce the VM_MAYWRITE check done at uffd registration
         * time.
         */
        if (!dst_vma->vm_userfaultfd_ctx.ctx)
                return false;

        return true;
}

static __always_inline
struct vm_area_struct *find_vma_and_prepare_anon(struct mm_struct *mm,
                                                 unsigned long addr)
{
        struct vm_area_struct *vma;

        mmap_assert_locked(mm);
        vma = vma_lookup(mm, addr);
        if (!vma)
                vma = ERR_PTR(-ENOENT);
        else if (!(vma->vm_flags & VM_SHARED) &&
                 unlikely(anon_vma_prepare(vma)))
                vma = ERR_PTR(-ENOMEM);

        return vma;
}

#ifdef CONFIG_PER_VMA_LOCK
/*
 * uffd_lock_vma() - Lookup and lock vma corresponding to @address.
 * @mm: mm to search vma in.
 * @address: address that the vma should contain.
 *
 * Should be called without holding mmap_lock.
 *
 * Return: A locked vma containing @address, -ENOENT if no vma is found, or
 * -ENOMEM if anon_vma couldn't be allocated.
 */
static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm,
                                       unsigned long address)
{
        struct vm_area_struct *vma;

        vma = lock_vma_under_rcu(mm, address);
        if (vma) {
                /*
                 * We know we're going to need to use anon_vma, so check
                 * that early.
                 */
                if (!(vma->vm_flags & VM_SHARED) && unlikely(!vma->anon_vma))
                        vma_end_read(vma);
                else
                        return vma;
        }

        mmap_read_lock(mm);
        vma = find_vma_and_prepare_anon(mm, address);
        if (!IS_ERR(vma)) {
                bool locked = vma_start_read_locked(vma);

                if (!locked)
                        vma = ERR_PTR(-EAGAIN);
        }

        mmap_read_unlock(mm);
        return vma;
}

static struct vm_area_struct *uffd_mfill_lock(struct mm_struct *dst_mm,
                                              unsigned long dst_start,
                                              unsigned long len)
{
        struct vm_area_struct *dst_vma;

        dst_vma = uffd_lock_vma(dst_mm, dst_start);
        if (IS_ERR(dst_vma) || validate_dst_vma(dst_vma, dst_start + len))
                return dst_vma;

        vma_end_read(dst_vma);
        return ERR_PTR(-ENOENT);
}

static void uffd_mfill_unlock(struct vm_area_struct *vma)
{
        vma_end_read(vma);
}

#else

static struct vm_area_struct *uffd_mfill_lock(struct mm_struct *dst_mm,
                                              unsigned long dst_start,
                                              unsigned long len)
{
        struct vm_area_struct *dst_vma;

        mmap_read_lock(dst_mm);
        dst_vma = find_vma_and_prepare_anon(dst_mm, dst_start);
        if (IS_ERR(dst_vma))
                goto out_unlock;

        if (validate_dst_vma(dst_vma, dst_start + len))
                return dst_vma;

        dst_vma = ERR_PTR(-ENOENT);
out_unlock:
        mmap_read_unlock(dst_mm);
        return dst_vma;
}

static void uffd_mfill_unlock(struct vm_area_struct *vma)
{
        mmap_read_unlock(vma->vm_mm);
}
#endif

/* Check if dst_addr is outside of file's size. Must be called with ptl held. */
static bool mfill_file_over_size(struct vm_area_struct *dst_vma,
                                 unsigned long dst_addr)
{
        struct inode *inode;
        pgoff_t offset, max_off;

        if (!dst_vma->vm_file)
                return false;

        inode = dst_vma->vm_file->f_inode;
        offset = linear_page_index(dst_vma, dst_addr);
        max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
        return offset >= max_off;
}

/*
 * Install PTEs, to map dst_addr (within dst_vma) to page.
 *
 * This function handles both MCOPY_ATOMIC_NORMAL and _CONTINUE for both shmem
 * and anon, and for both shared and private VMAs.
 */
int mfill_atomic_install_pte(pmd_t *dst_pmd,
                             struct vm_area_struct *dst_vma,
                             unsigned long dst_addr, struct page *page,
                             bool newly_allocated, uffd_flags_t flags)
{
        int ret;
        struct mm_struct *dst_mm = dst_vma->vm_mm;
        pte_t _dst_pte, *dst_pte;
        bool writable = dst_vma->vm_flags & VM_WRITE;
        bool vm_shared = dst_vma->vm_flags & VM_SHARED;
        spinlock_t *ptl;
        struct folio *folio = page_folio(page);
        bool page_in_cache = folio_mapping(folio);
        pte_t dst_ptep;

        _dst_pte = mk_pte(page, dst_vma->vm_page_prot);
        _dst_pte = pte_mkdirty(_dst_pte);
        if (page_in_cache && !vm_shared)
                writable = false;
        if (writable)
                _dst_pte = pte_mkwrite(_dst_pte, dst_vma);
        if (flags & MFILL_ATOMIC_WP)
                _dst_pte = pte_mkuffd_wp(_dst_pte);

        ret = -EAGAIN;
        dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
        if (!dst_pte)
                goto out;

        if (mfill_file_over_size(dst_vma, dst_addr)) {
                ret = -EFAULT;
                goto out_unlock;
        }

        ret = -EEXIST;

        dst_ptep = ptep_get(dst_pte);

        /*
         * We are allowed to overwrite a UFFD pte marker: consider when both
         * MISSING|WP registered, we firstly wr-protect a none pte which has no
         * page cache page backing it, then access the page.
         */
        if (!pte_none(dst_ptep) && !pte_is_uffd_marker(dst_ptep))
                goto out_unlock;

        if (page_in_cache) {
                /* Usually, cache pages are already added to LRU */
                if (newly_allocated)
                        folio_add_lru(folio);
                folio_add_file_rmap_pte(folio, page, dst_vma);
        } else {
                folio_add_new_anon_rmap(folio, dst_vma, dst_addr, RMAP_EXCLUSIVE);
                folio_add_lru_vma(folio, dst_vma);
        }

        /*
         * Must happen after rmap, as mm_counter() checks mapping (via
         * PageAnon()), which is set by __page_set_anon_rmap().
         */
        inc_mm_counter(dst_mm, mm_counter(folio));

        set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);

        /* No need to invalidate - it was non-present before */
        update_mmu_cache(dst_vma, dst_addr, dst_pte);
        ret = 0;
out_unlock:
        pte_unmap_unlock(dst_pte, ptl);
out:
        return ret;
}

static int mfill_atomic_pte_copy(pmd_t *dst_pmd,
                                 struct vm_area_struct *dst_vma,
                                 unsigned long dst_addr,
                                 unsigned long src_addr,
                                 uffd_flags_t flags,
                                 struct folio **foliop)
{
        void *kaddr;
        int ret;
        struct folio *folio;

        if (!*foliop) {
                ret = -ENOMEM;
                folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, dst_vma,
                                        dst_addr);
                if (!folio)
                        goto out;

                kaddr = kmap_local_folio(folio, 0);
                /*
                 * The read mmap_lock is held here.  Despite the
                 * mmap_lock being read recursive a deadlock is still
                 * possible if a writer has taken a lock.  For example:
                 *
                 * process A thread 1 takes read lock on own mmap_lock
                 * process A thread 2 calls mmap, blocks taking write lock
                 * process B thread 1 takes page fault, read lock on own mmap lock
                 * process B thread 2 calls mmap, blocks taking write lock
                 * process A thread 1 blocks taking read lock on process B
                 * process B thread 1 blocks taking read lock on process A
                 *
                 * Disable page faults to prevent potential deadlock
                 * and retry the copy outside the mmap_lock.
                 */
                pagefault_disable();
                ret = copy_from_user(kaddr, (const void __user *) src_addr,
                                     PAGE_SIZE);
                pagefault_enable();
                kunmap_local(kaddr);

                /* fallback to copy_from_user outside mmap_lock */
                if (unlikely(ret)) {
                        ret = -ENOENT;
                        *foliop = folio;
                        /* don't free the page */
                        goto out;
                }

                flush_dcache_folio(folio);
        } else {
                folio = *foliop;
                *foliop = NULL;
        }

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

        ret = -ENOMEM;
        if (mem_cgroup_charge(folio, dst_vma->vm_mm, GFP_KERNEL))
                goto out_release;

        ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
                                       &folio->page, true, flags);
        if (ret)
                goto out_release;
out:
        return ret;
out_release:
        folio_put(folio);
        goto out;
}

static int mfill_atomic_pte_zeroed_folio(pmd_t *dst_pmd,
                                         struct vm_area_struct *dst_vma,
                                         unsigned long dst_addr)
{
        struct folio *folio;
        int ret = -ENOMEM;

        folio = vma_alloc_zeroed_movable_folio(dst_vma, dst_addr);
        if (!folio)
                return ret;

        if (mem_cgroup_charge(folio, dst_vma->vm_mm, GFP_KERNEL))
                goto out_put;

        /*
         * The memory barrier inside __folio_mark_uptodate makes sure that
         * zeroing out the folio become visible before mapping the page
         * using set_pte_at(). See do_anonymous_page().
         */
        __folio_mark_uptodate(folio);

        ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
                                       &folio->page, true, 0);
        if (ret)
                goto out_put;

        return 0;
out_put:
        folio_put(folio);
        return ret;
}

static int mfill_atomic_pte_zeropage(pmd_t *dst_pmd,
                                     struct vm_area_struct *dst_vma,
                                     unsigned long dst_addr)
{
        pte_t _dst_pte, *dst_pte;
        spinlock_t *ptl;
        int ret;

        if (mm_forbids_zeropage(dst_vma->vm_mm))
                return mfill_atomic_pte_zeroed_folio(dst_pmd, dst_vma, dst_addr);

        _dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
                                         dst_vma->vm_page_prot));
        ret = -EAGAIN;
        dst_pte = pte_offset_map_lock(dst_vma->vm_mm, dst_pmd, dst_addr, &ptl);
        if (!dst_pte)
                goto out;
        if (mfill_file_over_size(dst_vma, dst_addr)) {
                ret = -EFAULT;
                goto out_unlock;
        }
        ret = -EEXIST;
        if (!pte_none(ptep_get(dst_pte)))
                goto out_unlock;
        set_pte_at(dst_vma->vm_mm, dst_addr, dst_pte, _dst_pte);
        /* No need to invalidate - it was non-present before */
        update_mmu_cache(dst_vma, dst_addr, dst_pte);
        ret = 0;
out_unlock:
        pte_unmap_unlock(dst_pte, ptl);
out:
        return ret;
}

/* Handles UFFDIO_CONTINUE for all shmem VMAs (shared or private). */
static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
                                     struct vm_area_struct *dst_vma,
                                     unsigned long dst_addr,
                                     uffd_flags_t flags)
{
        struct inode *inode = file_inode(dst_vma->vm_file);
        pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
        struct folio *folio;
        struct page *page;
        int ret;

        ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
        /* Our caller expects us to return -EFAULT if we failed to find folio */
        if (ret == -ENOENT)
                ret = -EFAULT;
        if (ret)
                goto out;
        if (!folio) {
                ret = -EFAULT;
                goto out;
        }

        page = folio_file_page(folio, pgoff);
        if (PageHWPoison(page)) {
                ret = -EIO;
                goto out_release;
        }

        ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
                                       page, false, flags);
        if (ret)
                goto out_release;

        folio_unlock(folio);
        ret = 0;
out:
        return ret;
out_release:
        folio_unlock(folio);
        folio_put(folio);
        goto out;
}

/* Handles UFFDIO_POISON for all non-hugetlb VMAs. */
static int mfill_atomic_pte_poison(pmd_t *dst_pmd,
                                   struct vm_area_struct *dst_vma,
                                   unsigned long dst_addr,
                                   uffd_flags_t flags)
{
        int ret;
        struct mm_struct *dst_mm = dst_vma->vm_mm;
        pte_t _dst_pte, *dst_pte;
        spinlock_t *ptl;

        _dst_pte = make_pte_marker(PTE_MARKER_POISONED);
        ret = -EAGAIN;
        dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
        if (!dst_pte)
                goto out;

        if (mfill_file_over_size(dst_vma, dst_addr)) {
                ret = -EFAULT;
                goto out_unlock;
        }

        ret = -EEXIST;
        /* Refuse to overwrite any PTE, even a PTE marker (e.g. UFFD WP). */
        if (!pte_none(ptep_get(dst_pte)))
                goto out_unlock;

        set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);

        /* No need to invalidate - it was non-present before */
        update_mmu_cache(dst_vma, dst_addr, dst_pte);
        ret = 0;
out_unlock:
        pte_unmap_unlock(dst_pte, ptl);
out:
        return ret;
}

static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
{
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;

        pgd = pgd_offset(mm, address);
        p4d = p4d_alloc(mm, pgd, address);
        if (!p4d)
                return NULL;
        pud = pud_alloc(mm, p4d, address);
        if (!pud)
                return NULL;
        /*
         * Note that we didn't run this because the pmd was
         * missing, the *pmd may be already established and in
         * turn it may also be a trans_huge_pmd.
         */
        return pmd_alloc(mm, pud, address);
}

#ifdef CONFIG_HUGETLB_PAGE
/*
 * mfill_atomic processing for HUGETLB vmas.  Note that this routine is
 * called with either vma-lock or mmap_lock held, it will release the lock
 * before returning.
 */
static __always_inline ssize_t mfill_atomic_hugetlb(
                                              struct userfaultfd_ctx *ctx,
                                              struct vm_area_struct *dst_vma,
                                              unsigned long dst_start,
                                              unsigned long src_start,
                                              unsigned long len,
                                              uffd_flags_t flags)
{
        struct mm_struct *dst_mm = dst_vma->vm_mm;
        ssize_t err;
        pte_t *dst_pte;
        unsigned long src_addr, dst_addr;
        long copied;
        struct folio *folio;
        unsigned long vma_hpagesize;
        pgoff_t idx;
        u32 hash;
        struct address_space *mapping;

        /*
         * There is no default zero huge page for all huge page sizes as
         * supported by hugetlb.  A PMD_SIZE huge pages may exist as used
         * by THP.  Since we can not reliably insert a zero page, this
         * feature is not supported.
         */
        if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
                up_read(&ctx->map_changing_lock);
                uffd_mfill_unlock(dst_vma);
                return -EINVAL;
        }

        src_addr = src_start;
        dst_addr = dst_start;
        copied = 0;
        folio = NULL;
        vma_hpagesize = vma_kernel_pagesize(dst_vma);

        /*
         * Validate alignment based on huge page size
         */
        err = -EINVAL;
        if (dst_start & (vma_hpagesize - 1) || len & (vma_hpagesize - 1))
                goto out_unlock;

retry:
        /*
         * On routine entry dst_vma is set.  If we had to drop mmap_lock and
         * retry, dst_vma will be set to NULL and we must lookup again.
         */
        if (!dst_vma) {
                dst_vma = uffd_mfill_lock(dst_mm, dst_start, len);
                if (IS_ERR(dst_vma)) {
                        err = PTR_ERR(dst_vma);
                        goto out;
                }

                err = -ENOENT;
                if (!is_vm_hugetlb_page(dst_vma))
                        goto out_unlock_vma;

                err = -EINVAL;
                if (vma_hpagesize != vma_kernel_pagesize(dst_vma))
                        goto out_unlock_vma;

                /*
                 * If memory mappings are changing because of non-cooperative
                 * operation (e.g. mremap) running in parallel, bail out and
                 * request the user to retry later
                 */
                down_read(&ctx->map_changing_lock);
                err = -EAGAIN;
                if (atomic_read(&ctx->mmap_changing))
                        goto out_unlock;
        }

        while (src_addr < src_start + len) {
                VM_WARN_ON_ONCE(dst_addr >= dst_start + len);

                /*
                 * Serialize via vma_lock and hugetlb_fault_mutex.
                 * vma_lock ensures the dst_pte remains valid even
                 * in the case of shared pmds.  fault mutex prevents
                 * races with other faulting threads.
                 */
                idx = linear_page_index(dst_vma, dst_addr);
                mapping = dst_vma->vm_file->f_mapping;
                hash = hugetlb_fault_mutex_hash(mapping, idx);
                mutex_lock(&hugetlb_fault_mutex_table[hash]);
                hugetlb_vma_lock_read(dst_vma);

                err = -ENOMEM;
                dst_pte = huge_pte_alloc(dst_mm, dst_vma, dst_addr, vma_hpagesize);
                if (!dst_pte) {
                        hugetlb_vma_unlock_read(dst_vma);
                        mutex_unlock(&hugetlb_fault_mutex_table[hash]);
                        goto out_unlock;
                }

                if (!uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) {
                        const pte_t ptep = huge_ptep_get(dst_mm, dst_addr, dst_pte);

                        if (!huge_pte_none(ptep) && !pte_is_uffd_marker(ptep)) {
                                err = -EEXIST;
                                hugetlb_vma_unlock_read(dst_vma);
                                mutex_unlock(&hugetlb_fault_mutex_table[hash]);
                                goto out_unlock;
                        }
                }

                err = hugetlb_mfill_atomic_pte(dst_pte, dst_vma, dst_addr,
                                               src_addr, flags, &folio);

                hugetlb_vma_unlock_read(dst_vma);
                mutex_unlock(&hugetlb_fault_mutex_table[hash]);

                cond_resched();

                if (unlikely(err == -ENOENT)) {
                        up_read(&ctx->map_changing_lock);
                        uffd_mfill_unlock(dst_vma);
                        VM_WARN_ON_ONCE(!folio);

                        err = copy_folio_from_user(folio,
                                                   (const void __user *)src_addr, true);
                        if (unlikely(err)) {
                                err = -EFAULT;
                                goto out;
                        }

                        dst_vma = NULL;
                        goto retry;
                } else
                        VM_WARN_ON_ONCE(folio);

                if (!err) {
                        dst_addr += vma_hpagesize;
                        src_addr += vma_hpagesize;
                        copied += vma_hpagesize;

                        if (fatal_signal_pending(current))
                                err = -EINTR;
                }
                if (err)
                        break;
        }

out_unlock:
        up_read(&ctx->map_changing_lock);
out_unlock_vma:
        uffd_mfill_unlock(dst_vma);
out:
        if (folio)
                folio_put(folio);
        VM_WARN_ON_ONCE(copied < 0);
        VM_WARN_ON_ONCE(err > 0);
        VM_WARN_ON_ONCE(!copied && !err);
        return copied ? copied : err;
}
#else /* !CONFIG_HUGETLB_PAGE */
/* fail at build time if gcc attempts to use this */
extern ssize_t mfill_atomic_hugetlb(struct userfaultfd_ctx *ctx,
                                    struct vm_area_struct *dst_vma,
                                    unsigned long dst_start,
                                    unsigned long src_start,
                                    unsigned long len,
                                    uffd_flags_t flags);
#endif /* CONFIG_HUGETLB_PAGE */

static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd,
                                                struct vm_area_struct *dst_vma,
                                                unsigned long dst_addr,
                                                unsigned long src_addr,
                                                uffd_flags_t flags,
                                                struct folio **foliop)
{
        ssize_t err;

        if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) {
                return mfill_atomic_pte_continue(dst_pmd, dst_vma,
                                                 dst_addr, flags);
        } else if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
                return mfill_atomic_pte_poison(dst_pmd, dst_vma,
                                               dst_addr, flags);
        }

        /*
         * The normal page fault path for a shmem will invoke the
         * fault, fill the hole in the file and COW it right away. The
         * result generates plain anonymous memory. So when we are
         * asked to fill an hole in a MAP_PRIVATE shmem mapping, we'll
         * generate anonymous memory directly without actually filling
         * the hole. For the MAP_PRIVATE case the robustness check
         * only happens in the pagetable (to verify it's still none)
         * and not in the radix tree.
         */
        if (!(dst_vma->vm_flags & VM_SHARED)) {
                if (uffd_flags_mode_is(flags, MFILL_ATOMIC_COPY))
                        err = mfill_atomic_pte_copy(dst_pmd, dst_vma,
                                                    dst_addr, src_addr,
                                                    flags, foliop);
                else
                        err = mfill_atomic_pte_zeropage(dst_pmd,
                                                 dst_vma, dst_addr);
        } else {
                err = shmem_mfill_atomic_pte(dst_pmd, dst_vma,
                                             dst_addr, src_addr,
                                             flags, foliop);
        }

        return err;
}

static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
                                            unsigned long dst_start,
                                            unsigned long src_start,
                                            unsigned long len,
                                            uffd_flags_t flags)
{
        struct mm_struct *dst_mm = ctx->mm;
        struct vm_area_struct *dst_vma;
        ssize_t err;
        pmd_t *dst_pmd;
        unsigned long src_addr, dst_addr;
        long copied;
        struct folio *folio;

        /*
         * Sanitize the command parameters:
         */
        VM_WARN_ON_ONCE(dst_start & ~PAGE_MASK);
        VM_WARN_ON_ONCE(len & ~PAGE_MASK);

        /* Does the address range wrap, or is the span zero-sized? */
        VM_WARN_ON_ONCE(src_start + len <= src_start);
        VM_WARN_ON_ONCE(dst_start + len <= dst_start);

        src_addr = src_start;
        dst_addr = dst_start;
        copied = 0;
        folio = NULL;
retry:
        /*
         * Make sure the vma is not shared, that the dst range is
         * both valid and fully within a single existing vma.
         */
        dst_vma = uffd_mfill_lock(dst_mm, dst_start, len);
        if (IS_ERR(dst_vma)) {
                err = PTR_ERR(dst_vma);
                goto out;
        }

        /*
         * If memory mappings are changing because of non-cooperative
         * operation (e.g. mremap) running in parallel, bail out and
         * request the user to retry later
         */
        down_read(&ctx->map_changing_lock);
        err = -EAGAIN;
        if (atomic_read(&ctx->mmap_changing))
                goto out_unlock;

        err = -EINVAL;
        /*
         * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
         * it will overwrite vm_ops, so vma_is_anonymous must return false.
         */
        if (WARN_ON_ONCE(vma_is_anonymous(dst_vma) &&
            dst_vma->vm_flags & VM_SHARED))
                goto out_unlock;

        /*
         * validate 'mode' now that we know the dst_vma: don't allow
         * a wrprotect copy if the userfaultfd didn't register as WP.
         */
        if ((flags & MFILL_ATOMIC_WP) && !(dst_vma->vm_flags & VM_UFFD_WP))
                goto out_unlock;

        /*
         * If this is a HUGETLB vma, pass off to appropriate routine
         */
        if (is_vm_hugetlb_page(dst_vma))
                return  mfill_atomic_hugetlb(ctx, dst_vma, dst_start,
                                             src_start, len, flags);

        if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
                goto out_unlock;
        if (!vma_is_shmem(dst_vma) &&
            uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
                goto out_unlock;

        while (src_addr < src_start + len) {
                pmd_t dst_pmdval;

                VM_WARN_ON_ONCE(dst_addr >= dst_start + len);

                dst_pmd = mm_alloc_pmd(dst_mm, dst_addr);
                if (unlikely(!dst_pmd)) {
                        err = -ENOMEM;
                        break;
                }

                dst_pmdval = pmdp_get_lockless(dst_pmd);
                if (unlikely(pmd_none(dst_pmdval)) &&
                    unlikely(__pte_alloc(dst_mm, dst_pmd))) {
                        err = -ENOMEM;
                        break;
                }
                dst_pmdval = pmdp_get_lockless(dst_pmd);
                /*
                 * If the dst_pmd is THP don't override it and just be strict.
                 * (This includes the case where the PMD used to be THP and
                 * changed back to none after __pte_alloc().)
                 */
                if (unlikely(!pmd_present(dst_pmdval) ||
                                pmd_trans_huge(dst_pmdval))) {
                        err = -EEXIST;
                        break;
                }
                if (unlikely(pmd_bad(dst_pmdval))) {
                        err = -EFAULT;
                        break;
                }
                /*
                 * For shmem mappings, khugepaged is allowed to remove page
                 * tables under us; pte_offset_map_lock() will deal with that.
                 */

                err = mfill_atomic_pte(dst_pmd, dst_vma, dst_addr,
                                       src_addr, flags, &folio);
                cond_resched();

                if (unlikely(err == -ENOENT)) {
                        void *kaddr;

                        up_read(&ctx->map_changing_lock);
                        uffd_mfill_unlock(dst_vma);
                        VM_WARN_ON_ONCE(!folio);

                        kaddr = kmap_local_folio(folio, 0);
                        err = copy_from_user(kaddr,
                                             (const void __user *) src_addr,
                                             PAGE_SIZE);
                        kunmap_local(kaddr);
                        if (unlikely(err)) {
                                err = -EFAULT;
                                goto out;
                        }
                        flush_dcache_folio(folio);
                        goto retry;
                } else
                        VM_WARN_ON_ONCE(folio);

                if (!err) {
                        dst_addr += PAGE_SIZE;
                        src_addr += PAGE_SIZE;
                        copied += PAGE_SIZE;

                        if (fatal_signal_pending(current))
                                err = -EINTR;
                }
                if (err)
                        break;
        }

out_unlock:
        up_read(&ctx->map_changing_lock);
        uffd_mfill_unlock(dst_vma);
out:
        if (folio)
                folio_put(folio);
        VM_WARN_ON_ONCE(copied < 0);
        VM_WARN_ON_ONCE(err > 0);
        VM_WARN_ON_ONCE(!copied && !err);
        return copied ? copied : err;
}

ssize_t mfill_atomic_copy(struct userfaultfd_ctx *ctx, unsigned long dst_start,
                          unsigned long src_start, unsigned long len,
                          uffd_flags_t flags)
{
        return mfill_atomic(ctx, dst_start, src_start, len,
                            uffd_flags_set_mode(flags, MFILL_ATOMIC_COPY));
}

ssize_t mfill_atomic_zeropage(struct userfaultfd_ctx *ctx,
                              unsigned long start,
                              unsigned long len)
{
        return mfill_atomic(ctx, start, 0, len,
                            uffd_flags_set_mode(0, MFILL_ATOMIC_ZEROPAGE));
}

ssize_t mfill_atomic_continue(struct userfaultfd_ctx *ctx, unsigned long start,
                              unsigned long len, uffd_flags_t flags)
{

        /*
         * A caller might reasonably assume that UFFDIO_CONTINUE contains an
         * smp_wmb() to ensure that any writes to the about-to-be-mapped page by
         * the thread doing the UFFDIO_CONTINUE are guaranteed to be visible to
         * subsequent loads from the page through the newly mapped address range.
         */
        smp_wmb();

        return mfill_atomic(ctx, start, 0, len,
                            uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
}

ssize_t mfill_atomic_poison(struct userfaultfd_ctx *ctx, unsigned long start,
                            unsigned long len, uffd_flags_t flags)
{
        return mfill_atomic(ctx, start, 0, len,
                            uffd_flags_set_mode(flags, MFILL_ATOMIC_POISON));
}

long uffd_wp_range(struct vm_area_struct *dst_vma,
                   unsigned long start, unsigned long len, bool enable_wp)
{
        unsigned int mm_cp_flags;
        struct mmu_gather tlb;
        long ret;

        VM_WARN_ONCE(start < dst_vma->vm_start || start + len > dst_vma->vm_end,
                        "The address range exceeds VMA boundary.\n");
        if (enable_wp)
                mm_cp_flags = MM_CP_UFFD_WP;
        else
                mm_cp_flags = MM_CP_UFFD_WP_RESOLVE;

        /*
         * vma->vm_page_prot already reflects that uffd-wp is enabled for this
         * VMA (see userfaultfd_set_vm_flags()) and that all PTEs are supposed
         * to be write-protected as default whenever protection changes.
         * Try upgrading write permissions manually.
         */
        if (!enable_wp && vma_wants_manual_pte_write_upgrade(dst_vma))
                mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
        tlb_gather_mmu(&tlb, dst_vma->vm_mm);
        ret = change_protection(&tlb, dst_vma, start, start + len, mm_cp_flags);
        tlb_finish_mmu(&tlb);

        return ret;
}

int mwriteprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
                        unsigned long len, bool enable_wp)
{
        struct mm_struct *dst_mm = ctx->mm;
        unsigned long end = start + len;
        unsigned long _start, _end;
        struct vm_area_struct *dst_vma;
        unsigned long page_mask;
        long err;
        VMA_ITERATOR(vmi, dst_mm, start);

        /*
         * Sanitize the command parameters:
         */
        VM_WARN_ON_ONCE(start & ~PAGE_MASK);
        VM_WARN_ON_ONCE(len & ~PAGE_MASK);

        /* Does the address range wrap, or is the span zero-sized? */
        VM_WARN_ON_ONCE(start + len <= start);

        mmap_read_lock(dst_mm);

        /*
         * If memory mappings are changing because of non-cooperative
         * operation (e.g. mremap) running in parallel, bail out and
         * request the user to retry later
         */
        down_read(&ctx->map_changing_lock);
        err = -EAGAIN;
        if (atomic_read(&ctx->mmap_changing))
                goto out_unlock;

        err = -ENOENT;
        for_each_vma_range(vmi, dst_vma, end) {

                if (!userfaultfd_wp(dst_vma)) {
                        err = -ENOENT;
                        break;
                }

                if (is_vm_hugetlb_page(dst_vma)) {
                        err = -EINVAL;
                        page_mask = vma_kernel_pagesize(dst_vma) - 1;
                        if ((start & page_mask) || (len & page_mask))
                                break;
                }

                _start = max(dst_vma->vm_start, start);
                _end = min(dst_vma->vm_end, end);

                err = uffd_wp_range(dst_vma, _start, _end - _start, enable_wp);

                /* Return 0 on success, <0 on failures */
                if (err < 0)
                        break;
                err = 0;
        }
out_unlock:
        up_read(&ctx->map_changing_lock);
        mmap_read_unlock(dst_mm);
        return err;
}


void double_pt_lock(spinlock_t *ptl1,
                    spinlock_t *ptl2)
        __acquires(ptl1)
        __acquires(ptl2)
{
        if (ptl1 > ptl2)
                swap(ptl1, ptl2);
        /* lock in virtual address order to avoid lock inversion */
        spin_lock(ptl1);
        if (ptl1 != ptl2)
                spin_lock_nested(ptl2, SINGLE_DEPTH_NESTING);
        else
                __acquire(ptl2);
}

void double_pt_unlock(spinlock_t *ptl1,
                      spinlock_t *ptl2)
        __releases(ptl1)
        __releases(ptl2)
{
        spin_unlock(ptl1);
        if (ptl1 != ptl2)
                spin_unlock(ptl2);
        else
                __release(ptl2);
}

static inline bool is_pte_pages_stable(pte_t *dst_pte, pte_t *src_pte,
                                       pte_t orig_dst_pte, pte_t orig_src_pte,
                                       pmd_t *dst_pmd, pmd_t dst_pmdval)
{
        return pte_same(ptep_get(src_pte), orig_src_pte) &&
               pte_same(ptep_get(dst_pte), orig_dst_pte) &&
               pmd_same(dst_pmdval, pmdp_get_lockless(dst_pmd));
}

/*
 * Checks if the two ptes and the corresponding folio are eligible for batched
 * move. If so, then returns pointer to the locked folio. Otherwise, returns NULL.
 *
 * NOTE: folio's reference is not required as the whole operation is within
 * PTL's critical section.
 */
static struct folio *check_ptes_for_batched_move(struct vm_area_struct *src_vma,
                                                 unsigned long src_addr,
                                                 pte_t *src_pte, pte_t *dst_pte)
{
        pte_t orig_dst_pte, orig_src_pte;
        struct folio *folio;

        orig_dst_pte = ptep_get(dst_pte);
        if (!pte_none(orig_dst_pte))
                return NULL;

        orig_src_pte = ptep_get(src_pte);
        if (!pte_present(orig_src_pte) || is_zero_pfn(pte_pfn(orig_src_pte)))
                return NULL;

        folio = vm_normal_folio(src_vma, src_addr, orig_src_pte);
        if (!folio || !folio_trylock(folio))
                return NULL;
        if (!PageAnonExclusive(&folio->page) || folio_test_large(folio)) {
                folio_unlock(folio);
                return NULL;
        }
        return folio;
}

/*
 * Moves src folios to dst in a batch as long as they are not large, and can
 * successfully take the lock via folio_trylock().
 */
static long move_present_ptes(struct mm_struct *mm,
                              struct vm_area_struct *dst_vma,
                              struct vm_area_struct *src_vma,
                              unsigned long dst_addr, unsigned long src_addr,
                              pte_t *dst_pte, pte_t *src_pte,
                              pte_t orig_dst_pte, pte_t orig_src_pte,
                              pmd_t *dst_pmd, pmd_t dst_pmdval,
                              spinlock_t *dst_ptl, spinlock_t *src_ptl,
                              struct folio **first_src_folio, unsigned long len)
{
        int err = 0;
        struct folio *src_folio = *first_src_folio;
        unsigned long src_start = src_addr;
        unsigned long src_end;

        len = pmd_addr_end(dst_addr, dst_addr + len) - dst_addr;
        src_end = pmd_addr_end(src_addr, src_addr + len);
        flush_cache_range(src_vma, src_addr, src_end);
        double_pt_lock(dst_ptl, src_ptl);

        if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
                                 dst_pmd, dst_pmdval)) {
                err = -EAGAIN;
                goto out;
        }
        if (folio_test_large(src_folio) ||
            folio_maybe_dma_pinned(src_folio) ||
            !PageAnonExclusive(&src_folio->page)) {
                err = -EBUSY;
                goto out;
        }
        /* It's safe to drop the reference now as the page-table is holding one. */
        folio_put(*first_src_folio);
        *first_src_folio = NULL;
        lazy_mmu_mode_enable();

        while (true) {
                orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
                /* Folio got pinned from under us. Put it back and fail the move. */
                if (folio_maybe_dma_pinned(src_folio)) {
                        set_pte_at(mm, src_addr, src_pte, orig_src_pte);
                        err = -EBUSY;
                        break;
                }

                folio_move_anon_rmap(src_folio, dst_vma);
                src_folio->index = linear_page_index(dst_vma, dst_addr);

                orig_dst_pte = folio_mk_pte(src_folio, dst_vma->vm_page_prot);
                /* Set soft dirty bit so userspace can notice the pte was moved */
                if (pgtable_supports_soft_dirty())
                        orig_dst_pte = pte_mksoft_dirty(orig_dst_pte);
                if (pte_dirty(orig_src_pte))
                        orig_dst_pte = pte_mkdirty(orig_dst_pte);
                orig_dst_pte = pte_mkwrite(orig_dst_pte, dst_vma);
                set_pte_at(mm, dst_addr, dst_pte, orig_dst_pte);

                src_addr += PAGE_SIZE;
                if (src_addr == src_end)
                        break;
                dst_addr += PAGE_SIZE;
                dst_pte++;
                src_pte++;

                folio_unlock(src_folio);
                src_folio = check_ptes_for_batched_move(src_vma, src_addr,
                                                        src_pte, dst_pte);
                if (!src_folio)
                        break;
        }

        lazy_mmu_mode_disable();
        if (src_addr > src_start)
                flush_tlb_range(src_vma, src_start, src_addr);

        if (src_folio)
                folio_unlock(src_folio);
out:
        double_pt_unlock(dst_ptl, src_ptl);
        return src_addr > src_start ? src_addr - src_start : err;
}

static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
                         unsigned long dst_addr, unsigned long src_addr,
                         pte_t *dst_pte, pte_t *src_pte,
                         pte_t orig_dst_pte, pte_t orig_src_pte,
                         pmd_t *dst_pmd, pmd_t dst_pmdval,
                         spinlock_t *dst_ptl, spinlock_t *src_ptl,
                         struct folio *src_folio,
                         struct swap_info_struct *si, swp_entry_t entry)
{
        /*
         * Check if the folio still belongs to the target swap entry after
         * acquiring the lock. Folio can be freed in the swap cache while
         * not locked.
         */
        if (src_folio && unlikely(!folio_test_swapcache(src_folio) ||
                                  entry.val != src_folio->swap.val))
                return -EAGAIN;

        double_pt_lock(dst_ptl, src_ptl);

        if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
                                 dst_pmd, dst_pmdval)) {
                double_pt_unlock(dst_ptl, src_ptl);
                return -EAGAIN;
        }

        /*
         * The src_folio resides in the swapcache, requiring an update to its
         * index and mapping to align with the dst_vma, where a swap-in may
         * occur and hit the swapcache after moving the PTE.
         */
        if (src_folio) {
                folio_move_anon_rmap(src_folio, dst_vma);
                src_folio->index = linear_page_index(dst_vma, dst_addr);
        } else {
                /*
                 * Check if the swap entry is cached after acquiring the src_pte
                 * lock. Otherwise, we might miss a newly loaded swap cache folio.
                 *
                 * We are trying to catch newly added swap cache, the only possible case is
                 * when a folio is swapped in and out again staying in swap cache, using the
                 * same entry before the PTE check above. The PTL is acquired and released
                 * twice, each time after updating the swap table. So holding
                 * the PTL here ensures we see the updated value.
                 */
                if (swap_cache_has_folio(entry)) {
                        double_pt_unlock(dst_ptl, src_ptl);
                        return -EAGAIN;
                }
        }

        orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
        if (pgtable_supports_soft_dirty())
                orig_src_pte = pte_swp_mksoft_dirty(orig_src_pte);
        set_pte_at(mm, dst_addr, dst_pte, orig_src_pte);
        double_pt_unlock(dst_ptl, src_ptl);

        return PAGE_SIZE;
}

static int move_zeropage_pte(struct mm_struct *mm,
                             struct vm_area_struct *dst_vma,
                             struct vm_area_struct *src_vma,
                             unsigned long dst_addr, unsigned long src_addr,
                             pte_t *dst_pte, pte_t *src_pte,
                             pte_t orig_dst_pte, pte_t orig_src_pte,
                             pmd_t *dst_pmd, pmd_t dst_pmdval,
                             spinlock_t *dst_ptl, spinlock_t *src_ptl)
{
        pte_t zero_pte;

        double_pt_lock(dst_ptl, src_ptl);
        if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
                                 dst_pmd, dst_pmdval)) {
                double_pt_unlock(dst_ptl, src_ptl);
                return -EAGAIN;
        }

        zero_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
                                         dst_vma->vm_page_prot));
        ptep_clear_flush(src_vma, src_addr, src_pte);
        set_pte_at(mm, dst_addr, dst_pte, zero_pte);
        double_pt_unlock(dst_ptl, src_ptl);

        return PAGE_SIZE;
}


/*
 * The mmap_lock for reading is held by the caller. Just move the page(s)
 * from src_pmd to dst_pmd if possible, and return number of bytes moved.
 * On failure, an error code is returned.
 */
static long move_pages_ptes(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
                            struct vm_area_struct *dst_vma,
                            struct vm_area_struct *src_vma,
                            unsigned long dst_addr, unsigned long src_addr,
                            unsigned long len, __u64 mode)
{
        struct swap_info_struct *si = NULL;
        pte_t orig_src_pte, orig_dst_pte;
        pte_t src_folio_pte;
        spinlock_t *src_ptl, *dst_ptl;
        pte_t *src_pte = NULL;
        pte_t *dst_pte = NULL;
        pmd_t dummy_pmdval;
        pmd_t dst_pmdval;
        struct folio *src_folio = NULL;
        struct mmu_notifier_range range;
        long ret = 0;

        mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm,
                                src_addr, src_addr + len);
        mmu_notifier_invalidate_range_start(&range);
retry:
        /*
         * Use the maywrite version to indicate that dst_pte will be modified,
         * since dst_pte needs to be none, the subsequent pte_same() check
         * cannot prevent the dst_pte page from being freed concurrently, so we
         * also need to obtain dst_pmdval and recheck pmd_same() later.
         */
        dst_pte = pte_offset_map_rw_nolock(mm, dst_pmd, dst_addr, &dst_pmdval,
                                           &dst_ptl);

        /* Retry if a huge pmd materialized from under us */
        if (unlikely(!dst_pte)) {
                ret = -EAGAIN;
                goto out;
        }

        /*
         * Unlike dst_pte, the subsequent pte_same() check can ensure the
         * stability of the src_pte page, so there is no need to get pmdval,
         * just pass a dummy variable to it.
         */
        src_pte = pte_offset_map_rw_nolock(mm, src_pmd, src_addr, &dummy_pmdval,
                                           &src_ptl);

        /*
         * We held the mmap_lock for reading so MADV_DONTNEED
         * can zap transparent huge pages under us, or the
         * transparent huge page fault can establish new
         * transparent huge pages under us.
         */
        if (unlikely(!src_pte)) {
                ret = -EAGAIN;
                goto out;
        }

        /* Sanity checks before the operation */
        if (pmd_none(*dst_pmd) || pmd_none(*src_pmd) ||
            pmd_trans_huge(*dst_pmd) || pmd_trans_huge(*src_pmd)) {
                ret = -EINVAL;
                goto out;
        }

        spin_lock(dst_ptl);
        orig_dst_pte = ptep_get(dst_pte);
        spin_unlock(dst_ptl);
        if (!pte_none(orig_dst_pte)) {
                ret = -EEXIST;
                goto out;
        }

        spin_lock(src_ptl);
        orig_src_pte = ptep_get(src_pte);
        spin_unlock(src_ptl);
        if (pte_none(orig_src_pte)) {
                if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES))
                        ret = -ENOENT;
                else /* nothing to do to move a hole */
                        ret = PAGE_SIZE;
                goto out;
        }

        /* If PTE changed after we locked the folio then start over */
        if (src_folio && unlikely(!pte_same(src_folio_pte, orig_src_pte))) {
                ret = -EAGAIN;
                goto out;
        }

        if (pte_present(orig_src_pte)) {
                if (is_zero_pfn(pte_pfn(orig_src_pte))) {
                        ret = move_zeropage_pte(mm, dst_vma, src_vma,
                                               dst_addr, src_addr, dst_pte, src_pte,
                                               orig_dst_pte, orig_src_pte,
                                               dst_pmd, dst_pmdval, dst_ptl, src_ptl);
                        goto out;
                }

                /*
                 * Pin and lock source folio. Since we are in RCU read section,
                 * we can't block, so on contention have to unmap the ptes,
                 * obtain the lock and retry.
                 */
                if (!src_folio) {
                        struct folio *folio;
                        bool locked;

                        /*
                         * Pin the page while holding the lock to be sure the
                         * page isn't freed under us
                         */
                        spin_lock(src_ptl);
                        if (!pte_same(orig_src_pte, ptep_get(src_pte))) {
                                spin_unlock(src_ptl);
                                ret = -EAGAIN;
                                goto out;
                        }

                        folio = vm_normal_folio(src_vma, src_addr, orig_src_pte);
                        if (!folio || !PageAnonExclusive(&folio->page)) {
                                spin_unlock(src_ptl);
                                ret = -EBUSY;
                                goto out;
                        }

                        locked = folio_trylock(folio);
                        /*
                         * We avoid waiting for folio lock with a raised
                         * refcount for large folios because extra refcounts
                         * will result in split_folio() failing later and
                         * retrying.  If multiple tasks are trying to move a
                         * large folio we can end up livelocking.
                         */
                        if (!locked && folio_test_large(folio)) {
                                spin_unlock(src_ptl);
                                ret = -EAGAIN;
                                goto out;
                        }

                        folio_get(folio);
                        src_folio = folio;
                        src_folio_pte = orig_src_pte;
                        spin_unlock(src_ptl);

                        if (!locked) {
                                pte_unmap(src_pte);
                                pte_unmap(dst_pte);
                                src_pte = dst_pte = NULL;
                                /* now we can block and wait */
                                folio_lock(src_folio);
                                goto retry;
                        }

                        if (WARN_ON_ONCE(!folio_test_anon(src_folio))) {
                                ret = -EBUSY;
                                goto out;
                        }
                }

                /* at this point we have src_folio locked */
                if (folio_test_large(src_folio)) {
                        /* split_folio() can block */
                        pte_unmap(src_pte);
                        pte_unmap(dst_pte);
                        src_pte = dst_pte = NULL;
                        ret = split_folio(src_folio);
                        if (ret)
                                goto out;
                        /* have to reacquire the folio after it got split */
                        folio_unlock(src_folio);
                        folio_put(src_folio);
                        src_folio = NULL;
                        goto retry;
                }

                ret = move_present_ptes(mm, dst_vma, src_vma,
                                        dst_addr, src_addr, dst_pte, src_pte,
                                        orig_dst_pte, orig_src_pte, dst_pmd,
                                        dst_pmdval, dst_ptl, src_ptl, &src_folio,
                                        len);
        } else { /* !pte_present() */
                struct folio *folio = NULL;
                const softleaf_t entry = softleaf_from_pte(orig_src_pte);

                if (softleaf_is_migration(entry)) {
                        pte_unmap(src_pte);
                        pte_unmap(dst_pte);
                        src_pte = dst_pte = NULL;
                        migration_entry_wait(mm, src_pmd, src_addr);

                        ret = -EAGAIN;
                        goto out;
                } else if (!softleaf_is_swap(entry)) {
                        ret = -EFAULT;
                        goto out;
                }

                if (!pte_swp_exclusive(orig_src_pte)) {
                        ret = -EBUSY;
                        goto out;
                }

                si = get_swap_device(entry);
                if (unlikely(!si)) {
                        ret = -EAGAIN;
                        goto out;
                }
                /*
                 * Verify the existence of the swapcache. If present, the folio's
                 * index and mapping must be updated even when the PTE is a swap
                 * entry. The anon_vma lock is not taken during this process since
                 * the folio has already been unmapped, and the swap entry is
                 * exclusive, preventing rmap walks.
                 *
                 * For large folios, return -EBUSY immediately, as split_folio()
                 * also returns -EBUSY when attempting to split unmapped large
                 * folios in the swapcache. This issue needs to be resolved
                 * separately to allow proper handling.
                 */
                if (!src_folio)
                        folio = swap_cache_get_folio(entry);
                if (folio) {
                        if (folio_test_large(folio)) {
                                ret = -EBUSY;
                                folio_put(folio);
                                goto out;
                        }
                        src_folio = folio;
                        src_folio_pte = orig_src_pte;
                        if (!folio_trylock(src_folio)) {
                                pte_unmap(src_pte);
                                pte_unmap(dst_pte);
                                src_pte = dst_pte = NULL;
                                put_swap_device(si);
                                si = NULL;
                                /* now we can block and wait */
                                folio_lock(src_folio);
                                goto retry;
                        }
                }
                ret = move_swap_pte(mm, dst_vma, dst_addr, src_addr, dst_pte, src_pte,
                                orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval,
                                dst_ptl, src_ptl, src_folio, si, entry);
        }

out:
        if (src_folio) {
                folio_unlock(src_folio);
                folio_put(src_folio);
        }
        /*
         * Unmap in reverse order (LIFO) to maintain proper kmap_local
         * index ordering when CONFIG_HIGHPTE is enabled. We mapped dst_pte
         * first, then src_pte, so we must unmap src_pte first, then dst_pte.
         */
        if (src_pte)
                pte_unmap(src_pte);
        if (dst_pte)
                pte_unmap(dst_pte);
        mmu_notifier_invalidate_range_end(&range);
        if (si)
                put_swap_device(si);

        return ret;
}

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
static inline bool move_splits_huge_pmd(unsigned long dst_addr,
                                        unsigned long src_addr,
                                        unsigned long src_end)
{
        return (src_addr & ~HPAGE_PMD_MASK) || (dst_addr & ~HPAGE_PMD_MASK) ||
                src_end - src_addr < HPAGE_PMD_SIZE;
}
#else
static inline bool move_splits_huge_pmd(unsigned long dst_addr,
                                        unsigned long src_addr,
                                        unsigned long src_end)
{
        /* This is unreachable anyway, just to avoid warnings when HPAGE_PMD_SIZE==0 */
        return false;
}
#endif

static inline bool vma_move_compatible(struct vm_area_struct *vma)
{
        return !(vma->vm_flags & (VM_PFNMAP | VM_IO |  VM_HUGETLB |
                                  VM_MIXEDMAP | VM_SHADOW_STACK));
}

static int validate_move_areas(struct userfaultfd_ctx *ctx,
                               struct vm_area_struct *src_vma,
                               struct vm_area_struct *dst_vma)
{
        /* Only allow moving if both have the same access and protection */
        if ((src_vma->vm_flags & VM_ACCESS_FLAGS) != (dst_vma->vm_flags & VM_ACCESS_FLAGS) ||
            pgprot_val(src_vma->vm_page_prot) != pgprot_val(dst_vma->vm_page_prot))
                return -EINVAL;

        /* Only allow moving if both are mlocked or both aren't */
        if ((src_vma->vm_flags & VM_LOCKED) != (dst_vma->vm_flags & VM_LOCKED))
                return -EINVAL;

        /*
         * For now, we keep it simple and only move between writable VMAs.
         * Access flags are equal, therefore checking only the source is enough.
         */
        if (!(src_vma->vm_flags & VM_WRITE))
                return -EINVAL;

        /* Check if vma flags indicate content which can be moved */
        if (!vma_move_compatible(src_vma) || !vma_move_compatible(dst_vma))
                return -EINVAL;

        /* Ensure dst_vma is registered in uffd we are operating on */
        if (!dst_vma->vm_userfaultfd_ctx.ctx ||
            dst_vma->vm_userfaultfd_ctx.ctx != ctx)
                return -EINVAL;

        /* Only allow moving across anonymous vmas */
        if (!vma_is_anonymous(src_vma) || !vma_is_anonymous(dst_vma))
                return -EINVAL;

        return 0;
}

static __always_inline
int find_vmas_mm_locked(struct mm_struct *mm,
                        unsigned long dst_start,
                        unsigned long src_start,
                        struct vm_area_struct **dst_vmap,
                        struct vm_area_struct **src_vmap)
{
        struct vm_area_struct *vma;

        mmap_assert_locked(mm);
        vma = find_vma_and_prepare_anon(mm, dst_start);
        if (IS_ERR(vma))
                return PTR_ERR(vma);

        *dst_vmap = vma;
        /* Skip finding src_vma if src_start is in dst_vma */
        if (src_start >= vma->vm_start && src_start < vma->vm_end)
                goto out_success;

        vma = vma_lookup(mm, src_start);
        if (!vma)
                return -ENOENT;
out_success:
        *src_vmap = vma;
        return 0;
}

#ifdef CONFIG_PER_VMA_LOCK
static int uffd_move_lock(struct mm_struct *mm,
                          unsigned long dst_start,
                          unsigned long src_start,
                          struct vm_area_struct **dst_vmap,
                          struct vm_area_struct **src_vmap)
{
        struct vm_area_struct *vma;
        int err;

        vma = uffd_lock_vma(mm, dst_start);
        if (IS_ERR(vma))
                return PTR_ERR(vma);

        *dst_vmap = vma;
        /*
         * Skip finding src_vma if src_start is in dst_vma. This also ensures
         * that we don't lock the same vma twice.
         */
        if (src_start >= vma->vm_start && src_start < vma->vm_end) {
                *src_vmap = vma;
                return 0;
        }

        /*
         * Using uffd_lock_vma() to get src_vma can lead to following deadlock:
         *
         * Thread1                              Thread2
         * -------                              -------
         * vma_start_read(dst_vma)
         *                                      mmap_write_lock(mm)
         *                                      vma_start_write(src_vma)
         * vma_start_read(src_vma)
         * mmap_read_lock(mm)
         *                                      vma_start_write(dst_vma)
         */
        *src_vmap = lock_vma_under_rcu(mm, src_start);
        if (likely(*src_vmap))
                return 0;

        /* Undo any locking and retry in mmap_lock critical section */
        vma_end_read(*dst_vmap);

        mmap_read_lock(mm);
        err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
        if (err)
                goto out;

        if (!vma_start_read_locked(*dst_vmap)) {
                err = -EAGAIN;
                goto out;
        }

        /* Nothing further to do if both vmas are locked. */
        if (*dst_vmap == *src_vmap)
                goto out;

        if (!vma_start_read_locked_nested(*src_vmap, SINGLE_DEPTH_NESTING)) {
                /* Undo dst_vmap locking if src_vmap failed to lock */
                vma_end_read(*dst_vmap);
                err = -EAGAIN;
        }
out:
        mmap_read_unlock(mm);
        return err;
}

static void uffd_move_unlock(struct vm_area_struct *dst_vma,
                             struct vm_area_struct *src_vma)
{
        vma_end_read(src_vma);
        if (src_vma != dst_vma)
                vma_end_read(dst_vma);
}

#else

static int uffd_move_lock(struct mm_struct *mm,
                          unsigned long dst_start,
                          unsigned long src_start,
                          struct vm_area_struct **dst_vmap,
                          struct vm_area_struct **src_vmap)
{
        int err;

        mmap_read_lock(mm);
        err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
        if (err)
                mmap_read_unlock(mm);
        return err;
}

static void uffd_move_unlock(struct vm_area_struct *dst_vma,
                             struct vm_area_struct *src_vma)
{
        mmap_assert_locked(src_vma->vm_mm);
        mmap_read_unlock(dst_vma->vm_mm);
}
#endif

/**
 * move_pages - move arbitrary anonymous pages of an existing vma
 * @ctx: pointer to the userfaultfd context
 * @dst_start: start of the destination virtual memory range
 * @src_start: start of the source virtual memory range
 * @len: length of the virtual memory range
 * @mode: flags from uffdio_move.mode
 *
 * It will either use the mmap_lock in read mode or per-vma locks
 *
 * move_pages() remaps arbitrary anonymous pages atomically in zero
 * copy. It only works on non shared anonymous pages because those can
 * be relocated without generating non linear anon_vmas in the rmap
 * code.
 *
 * It provides a zero copy mechanism to handle userspace page faults.
 * The source vma pages should have mapcount == 1, which can be
 * enforced by using madvise(MADV_DONTFORK) on src vma.
 *
 * The thread receiving the page during the userland page fault
 * will receive the faulting page in the source vma through the network,
 * storage or any other I/O device (MADV_DONTFORK in the source vma
 * avoids move_pages() to fail with -EBUSY if the process forks before
 * move_pages() is called), then it will call move_pages() to map the
 * page in the faulting address in the destination vma.
 *
 * This userfaultfd command works purely via pagetables, so it's the
 * most efficient way to move physical non shared anonymous pages
 * across different virtual addresses. Unlike mremap()/mmap()/munmap()
 * it does not create any new vmas. The mapping in the destination
 * address is atomic.
 *
 * It only works if the vma protection bits are identical from the
 * source and destination vma.
 *
 * It can remap non shared anonymous pages within the same vma too.
 *
 * If the source virtual memory range has any unmapped holes, or if
 * the destination virtual memory range is not a whole unmapped hole,
 * move_pages() will fail respectively with -ENOENT or -EEXIST. This
 * provides a very strict behavior to avoid any chance of memory
 * corruption going unnoticed if there are userland race conditions.
 * Only one thread should resolve the userland page fault at any given
 * time for any given faulting address. This means that if two threads
 * try to both call move_pages() on the same destination address at the
 * same time, the second thread will get an explicit error from this
 * command.
 *
 * The command retval will return "len" is successful. The command
 * however can be interrupted by fatal signals or errors. If
 * interrupted it will return the number of bytes successfully
 * remapped before the interruption if any, or the negative error if
 * none. It will never return zero. Either it will return an error or
 * an amount of bytes successfully moved. If the retval reports a
 * "short" remap, the move_pages() command should be repeated by
 * userland with src+retval, dst+reval, len-retval if it wants to know
 * about the error that interrupted it.
 *
 * The UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES flag can be specified to
 * prevent -ENOENT errors to materialize if there are holes in the
 * source virtual range that is being remapped. The holes will be
 * accounted as successfully remapped in the retval of the
 * command. This is mostly useful to remap hugepage naturally aligned
 * virtual regions without knowing if there are transparent hugepage
 * in the regions or not, but preventing the risk of having to split
 * the hugepmd during the remap.
 */
ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
                   unsigned long src_start, unsigned long len, __u64 mode)
{
        struct mm_struct *mm = ctx->mm;
        struct vm_area_struct *src_vma, *dst_vma;
        unsigned long src_addr, dst_addr, src_end;
        pmd_t *src_pmd, *dst_pmd;
        long err = -EINVAL;
        ssize_t moved = 0;

        /* Sanitize the command parameters. */
        VM_WARN_ON_ONCE(src_start & ~PAGE_MASK);
        VM_WARN_ON_ONCE(dst_start & ~PAGE_MASK);
        VM_WARN_ON_ONCE(len & ~PAGE_MASK);

        /* Does the address range wrap, or is the span zero-sized? */
        VM_WARN_ON_ONCE(src_start + len < src_start);
        VM_WARN_ON_ONCE(dst_start + len < dst_start);

        err = uffd_move_lock(mm, dst_start, src_start, &dst_vma, &src_vma);
        if (err)
                goto out;

        /* Re-check after taking map_changing_lock */
        err = -EAGAIN;
        down_read(&ctx->map_changing_lock);
        if (likely(atomic_read(&ctx->mmap_changing)))
                goto out_unlock;
        /*
         * Make sure the vma is not shared, that the src and dst remap
         * ranges are both valid and fully within a single existing
         * vma.
         */
        err = -EINVAL;
        if (src_vma->vm_flags & VM_SHARED)
                goto out_unlock;
        if (src_start + len > src_vma->vm_end)
                goto out_unlock;

        if (dst_vma->vm_flags & VM_SHARED)
                goto out_unlock;
        if (dst_start + len > dst_vma->vm_end)
                goto out_unlock;

        err = validate_move_areas(ctx, src_vma, dst_vma);
        if (err)
                goto out_unlock;

        for (src_addr = src_start, dst_addr = dst_start, src_end = src_start + len;
             src_addr < src_end;) {
                spinlock_t *ptl;
                pmd_t dst_pmdval;
                unsigned long step_size;

                /*
                 * Below works because anonymous area would not have a
                 * transparent huge PUD. If file-backed support is added,
                 * that case would need to be handled here.
                 */
                src_pmd = mm_find_pmd(mm, src_addr);
                if (unlikely(!src_pmd)) {
                        if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) {
                                err = -ENOENT;
                                break;
                        }
                        src_pmd = mm_alloc_pmd(mm, src_addr);
                        if (unlikely(!src_pmd)) {
                                err = -ENOMEM;
                                break;
                        }
                }
                dst_pmd = mm_alloc_pmd(mm, dst_addr);
                if (unlikely(!dst_pmd)) {
                        err = -ENOMEM;
                        break;
                }

                dst_pmdval = pmdp_get_lockless(dst_pmd);
                /*
                 * If the dst_pmd is mapped as THP don't override it and just
                 * be strict. If dst_pmd changes into TPH after this check, the
                 * move_pages_huge_pmd() will detect the change and retry
                 * while move_pages_pte() will detect the change and fail.
                 */
                if (unlikely(pmd_trans_huge(dst_pmdval))) {
                        err = -EEXIST;
                        break;
                }

                ptl = pmd_trans_huge_lock(src_pmd, src_vma);
                if (ptl) {
                        /* Check if we can move the pmd without splitting it. */
                        if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
                            !pmd_none(dst_pmdval)) {
                                /* Can be a migration entry */
                                if (pmd_present(*src_pmd)) {
                                        struct folio *folio = pmd_folio(*src_pmd);

                                        if (!is_huge_zero_folio(folio) &&
                                            !PageAnonExclusive(&folio->page)) {
                                                spin_unlock(ptl);
                                                err = -EBUSY;
                                                break;
                                        }
                                }

                                spin_unlock(ptl);
                                split_huge_pmd(src_vma, src_pmd, src_addr);
                                /* The folio will be split by move_pages_pte() */
                                continue;
                        }

                        err = move_pages_huge_pmd(mm, dst_pmd, src_pmd,
                                                  dst_pmdval, dst_vma, src_vma,
                                                  dst_addr, src_addr);
                        step_size = HPAGE_PMD_SIZE;
                } else {
                        long ret;

                        if (pmd_none(*src_pmd)) {
                                if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) {
                                        err = -ENOENT;
                                        break;
                                }
                                if (unlikely(__pte_alloc(mm, src_pmd))) {
                                        err = -ENOMEM;
                                        break;
                                }
                        }

                        if (unlikely(pte_alloc(mm, dst_pmd))) {
                                err = -ENOMEM;
                                break;
                        }

                        ret = move_pages_ptes(mm, dst_pmd, src_pmd,
                                              dst_vma, src_vma, dst_addr,
                                              src_addr, src_end - src_addr, mode);
                        if (ret < 0)
                                err = ret;
                        else
                                step_size = ret;
                }

                cond_resched();

                if (fatal_signal_pending(current)) {
                        /* Do not override an error */
                        if (!err || err == -EAGAIN)
                                err = -EINTR;
                        break;
                }

                if (err) {
                        if (err == -EAGAIN)
                                continue;
                        break;
                }

                /* Proceed to the next page */
                dst_addr += step_size;
                src_addr += step_size;
                moved += step_size;
        }

out_unlock:
        up_read(&ctx->map_changing_lock);
        uffd_move_unlock(dst_vma, src_vma);
out:
        VM_WARN_ON_ONCE(moved < 0);
        VM_WARN_ON_ONCE(err > 0);
        VM_WARN_ON_ONCE(!moved && !err);
        return moved ? moved : err;
}

static void userfaultfd_set_vm_flags(struct vm_area_struct *vma,
                                     vm_flags_t vm_flags)
{
        const bool uffd_wp_changed = (vma->vm_flags ^ vm_flags) & VM_UFFD_WP;

        vm_flags_reset(vma, vm_flags);
        /*
         * For shared mappings, we want to enable writenotify while
         * userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply
         * recalculate vma->vm_page_prot whenever userfaultfd-wp changes.
         */
        if ((vma->vm_flags & VM_SHARED) && uffd_wp_changed)
                vma_set_page_prot(vma);
}

static void userfaultfd_set_ctx(struct vm_area_struct *vma,
                                struct userfaultfd_ctx *ctx,
                                vm_flags_t vm_flags)
{
        vma_start_write(vma);
        vma->vm_userfaultfd_ctx = (struct vm_userfaultfd_ctx){ctx};
        userfaultfd_set_vm_flags(vma,
                                 (vma->vm_flags & ~__VM_UFFD_FLAGS) | vm_flags);
}

void userfaultfd_reset_ctx(struct vm_area_struct *vma)
{
        userfaultfd_set_ctx(vma, NULL, 0);
}

struct vm_area_struct *userfaultfd_clear_vma(struct vma_iterator *vmi,
                                             struct vm_area_struct *prev,
                                             struct vm_area_struct *vma,
                                             unsigned long start,
                                             unsigned long end)
{
        struct vm_area_struct *ret;
        bool give_up_on_oom = false;

        /*
         * If we are modifying only and not splitting, just give up on the merge
         * if OOM prevents us from merging successfully.
         */
        if (start == vma->vm_start && end == vma->vm_end)
                give_up_on_oom = true;

        /* Reset ptes for the whole vma range if wr-protected */
        if (userfaultfd_wp(vma))
                uffd_wp_range(vma, start, end - start, false);

        ret = vma_modify_flags_uffd(vmi, prev, vma, start, end,
                                    vma->vm_flags & ~__VM_UFFD_FLAGS,
                                    NULL_VM_UFFD_CTX, give_up_on_oom);

        /*
         * In the vma_merge() successful mprotect-like case 8:
         * the next vma was merged into the current one and
         * the current one has not been updated yet.
         */
        if (!IS_ERR(ret))
                userfaultfd_reset_ctx(ret);

        return ret;
}

/* Assumes mmap write lock taken, and mm_struct pinned. */
int userfaultfd_register_range(struct userfaultfd_ctx *ctx,
                               struct vm_area_struct *vma,
                               vm_flags_t vm_flags,
                               unsigned long start, unsigned long end,
                               bool wp_async)
{
        VMA_ITERATOR(vmi, ctx->mm, start);
        struct vm_area_struct *prev = vma_prev(&vmi);
        unsigned long vma_end;
        vm_flags_t new_flags;

        if (vma->vm_start < start)
                prev = vma;

        for_each_vma_range(vmi, vma, end) {
                cond_resched();

                VM_WARN_ON_ONCE(!vma_can_userfault(vma, vm_flags, wp_async));
                VM_WARN_ON_ONCE(vma->vm_userfaultfd_ctx.ctx &&
                                vma->vm_userfaultfd_ctx.ctx != ctx);
                VM_WARN_ON_ONCE(!(vma->vm_flags & VM_MAYWRITE));

                /*
                 * Nothing to do: this vma is already registered into this
                 * userfaultfd and with the right tracking mode too.
                 */
                if (vma->vm_userfaultfd_ctx.ctx == ctx &&
                    (vma->vm_flags & vm_flags) == vm_flags)
                        goto skip;

                if (vma->vm_start > start)
                        start = vma->vm_start;
                vma_end = min(end, vma->vm_end);

                new_flags = (vma->vm_flags & ~__VM_UFFD_FLAGS) | vm_flags;
                vma = vma_modify_flags_uffd(&vmi, prev, vma, start, vma_end,
                                            new_flags,
                                            (struct vm_userfaultfd_ctx){ctx},
                                            /* give_up_on_oom = */false);
                if (IS_ERR(vma))
                        return PTR_ERR(vma);

                /*
                 * In the vma_merge() successful mprotect-like case 8:
                 * the next vma was merged into the current one and
                 * the current one has not been updated yet.
                 */
                userfaultfd_set_ctx(vma, ctx, vm_flags);

                if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma))
                        hugetlb_unshare_all_pmds(vma);

skip:
                prev = vma;
                start = vma->vm_end;
        }

        return 0;
}

void userfaultfd_release_new(struct userfaultfd_ctx *ctx)
{
        struct mm_struct *mm = ctx->mm;
        struct vm_area_struct *vma;
        VMA_ITERATOR(vmi, mm, 0);

        /* the various vma->vm_userfaultfd_ctx still points to it */
        mmap_write_lock(mm);
        for_each_vma(vmi, vma) {
                if (vma->vm_userfaultfd_ctx.ctx == ctx)
                        userfaultfd_reset_ctx(vma);
        }
        mmap_write_unlock(mm);
}

void userfaultfd_release_all(struct mm_struct *mm,
                             struct userfaultfd_ctx *ctx)
{
        struct vm_area_struct *vma, *prev;
        VMA_ITERATOR(vmi, mm, 0);

        if (!mmget_not_zero(mm))
                return;

        /*
         * Flush page faults out of all CPUs. NOTE: all page faults
         * must be retried without returning VM_FAULT_SIGBUS if
         * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx
         * changes while handle_userfault released the mmap_lock. So
         * it's critical that released is set to true (above), before
         * taking the mmap_lock for writing.
         */
        mmap_write_lock(mm);
        prev = NULL;
        for_each_vma(vmi, vma) {
                cond_resched();
                VM_WARN_ON_ONCE(!!vma->vm_userfaultfd_ctx.ctx ^
                                !!(vma->vm_flags & __VM_UFFD_FLAGS));
                if (vma->vm_userfaultfd_ctx.ctx != ctx) {
                        prev = vma;
                        continue;
                }

                vma = userfaultfd_clear_vma(&vmi, prev, vma,
                                            vma->vm_start, vma->vm_end);
                prev = vma;
        }
        mmap_write_unlock(mm);
        mmput(mm);
}