mm/userfaultfd.c - linux - Git at Google

 // 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/swapops.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"

 static __always_inline
 struct vm_area_struct *find_dst_vma(struct mm_struct *dst_mm,
 				    unsigned long dst_start,
 				    unsigned long len)
 {
 	/*
 	 * Make sure that the dst range is both valid and fully within a
 	 * single existing vma.
 	 */
 	struct vm_area_struct *dst_vma;

 	dst_vma = find_vma(dst_mm, dst_start);
 	if (!range_in_vma(dst_vma, dst_start, dst_start + len))
 		return NULL;

 	/*
 	 * 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 NULL;

 	return dst_vma;
 }

 /*
  * 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;
 	bool page_in_cache = page_mapping(page);
 	spinlock_t *ptl;
 	struct folio *folio;
 	struct inode *inode;
 	pgoff_t offset, max_off;

 	_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);
 	if (flags & MFILL_ATOMIC_WP)
 		_dst_pte = pte_mkuffd_wp(_dst_pte);

 	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);

 	if (vma_is_shmem(dst_vma)) {
 		/* serialize against truncate with the page table lock */
 		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);
 		ret = -EFAULT;
 		if (unlikely(offset >= max_off))
 			goto out_unlock;
 	}

 	ret = -EEXIST;
 	/*
 	 * We allow to overwrite a 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_mostly(*dst_pte))
 		goto out_unlock;

 	folio = page_folio(page);
 	if (page_in_cache) {
 		/* Usually, cache pages are already added to LRU */
 		if (newly_allocated)
 			folio_add_lru(folio);
 		page_add_file_rmap(page, dst_vma, false);
 	} else {
 		page_add_new_anon_rmap(page, dst_vma, dst_addr);
 		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(page));

 	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);
 	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, false);
 		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_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;
 	pgoff_t offset, max_off;
 	struct inode *inode;

 	_dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
 					 dst_vma->vm_page_prot));
 	dst_pte = pte_offset_map_lock(dst_vma->vm_mm, dst_pmd, dst_addr, &ptl);
 	if (dst_vma->vm_file) {
 		/* the shmem MAP_PRIVATE case requires checking the i_size */
 		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);
 		ret = -EFAULT;
 		if (unlikely(offset >= max_off))
 			goto out_unlock;
 	}
 	ret = -EEXIST;
 	if (!pte_none(*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);
 	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, &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;
 }

 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 mmap_lock held, it will release mmap_lock before returning.
  */
 static __always_inline ssize_t mfill_atomic_hugetlb(
 					      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;
 	int vm_shared = dst_vma->vm_flags & VM_SHARED;
 	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)) {
 		mmap_read_unlock(dst_mm);
 		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) {
 		err = -ENOENT;
 		dst_vma = find_dst_vma(dst_mm, dst_start, len);
 		if (!dst_vma || !is_vm_hugetlb_page(dst_vma))
 			goto out_unlock;

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

 		vm_shared = dst_vma->vm_flags & VM_SHARED;
 	}

 	/*
 	 * If not shared, ensure the dst_vma has a anon_vma.
 	 */
 	err = -ENOMEM;
 	if (!vm_shared) {
 		if (unlikely(anon_vma_prepare(dst_vma)))
 			goto out_unlock;
 	}

 	while (src_addr < src_start + len) {
 		BUG_ON(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) &&
 		    !huge_pte_none_mostly(huge_ptep_get(dst_pte))) {
 			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)) {
 			mmap_read_unlock(dst_mm);
 			BUG_ON(!folio);

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

 			dst_vma = NULL;
 			goto retry;
 		} else
 			BUG_ON(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:
 	mmap_read_unlock(dst_mm);
 out:
 	if (folio)
 		folio_put(folio);
 	BUG_ON(copied < 0);
 	BUG_ON(err > 0);
 	BUG_ON(!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 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);
 	}

 	/*
 	 * 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 mm_struct *dst_mm,
 					    unsigned long dst_start,
 					    unsigned long src_start,
 					    unsigned long len,
 					    atomic_t *mmap_changing,
 					    uffd_flags_t flags)
 {
 	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:
 	 */
 	BUG_ON(dst_start & ~PAGE_MASK);
 	BUG_ON(len & ~PAGE_MASK);

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

 	src_addr = src_start;
 	dst_addr = dst_start;
 	copied = 0;
 	folio = NULL;
 retry:
 	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
 	 */
 	err = -EAGAIN;
 	if (mmap_changing && atomic_read(mmap_changing))
 		goto out_unlock;

 	/*
 	 * Make sure the vma is not shared, that the dst range is
 	 * both valid and fully within a single existing vma.
 	 */
 	err = -ENOENT;
 	dst_vma = find_dst_vma(dst_mm, dst_start, len);
 	if (!dst_vma)
 		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(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;

 	/*
 	 * Ensure the dst_vma has a anon_vma or this page
 	 * would get a NULL anon_vma when moved in the
 	 * dst_vma.
 	 */
 	err = -ENOMEM;
 	if (!(dst_vma->vm_flags & VM_SHARED) &&
 	    unlikely(anon_vma_prepare(dst_vma)))
 		goto out_unlock;

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

 		BUG_ON(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 the dst_pmd is mapped as THP don't
 		 * override it and just be strict.
 		 */
 		if (unlikely(pmd_trans_huge(dst_pmdval))) {
 			err = -EEXIST;
 			break;
 		}
 		if (unlikely(pmd_none(dst_pmdval)) &&
 		    unlikely(__pte_alloc(dst_mm, dst_pmd))) {
 			err = -ENOMEM;
 			break;
 		}
 		/* If an huge pmd materialized from under us fail */
 		if (unlikely(pmd_trans_huge(*dst_pmd))) {
 			err = -EFAULT;
 			break;
 		}

 		BUG_ON(pmd_none(*dst_pmd));
 		BUG_ON(pmd_trans_huge(*dst_pmd));

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

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

 			mmap_read_unlock(dst_mm);
 			BUG_ON(!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
 			BUG_ON(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:
 	mmap_read_unlock(dst_mm);
 out:
 	if (folio)
 		folio_put(folio);
 	BUG_ON(copied < 0);
 	BUG_ON(err > 0);
 	BUG_ON(!copied && !err);
 	return copied ? copied : err;
 }

 ssize_t mfill_atomic_copy(struct mm_struct *dst_mm, unsigned long dst_start,
 			  unsigned long src_start, unsigned long len,
 			  atomic_t *mmap_changing, uffd_flags_t flags)
 {
 	return mfill_atomic(dst_mm, dst_start, src_start, len, mmap_changing,
 			    uffd_flags_set_mode(flags, MFILL_ATOMIC_COPY));
 }

 ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm, unsigned long start,
 			      unsigned long len, atomic_t *mmap_changing)
 {
 	return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
 			    uffd_flags_set_mode(0, MFILL_ATOMIC_ZEROPAGE));
 }

 ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long start,
 			      unsigned long len, atomic_t *mmap_changing,
 			      uffd_flags_t flags)
 {
 	return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
 			    uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
 }

 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 mm_struct *dst_mm, unsigned long start,
 			unsigned long len, bool enable_wp,
 			atomic_t *mmap_changing)
 {
 	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:
 	 */
 	BUG_ON(start & ~PAGE_MASK);
 	BUG_ON(len & ~PAGE_MASK);

 	/* Does the address range wrap, or is the span zero-sized? */
 	BUG_ON(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
 	 */
 	err = -EAGAIN;
 	if (mmap_changing && atomic_read(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:
 	mmap_read_unlock(dst_mm);
 	return err;
 }
	// 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/swapops.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"

	static __always_inline
	struct vm_area_struct find_dst_vma(struct mm_struct dst_mm,
	unsigned long dst_start,
	unsigned long len)
	{
	/*
	* Make sure that the dst range is both valid and fully within a
	* single existing vma.
	*/
	struct vm_area_struct *dst_vma;

	dst_vma = find_vma(dst_mm, dst_start);
	if (!range_in_vma(dst_vma, dst_start, dst_start + len))
	return NULL;

	/*
	* 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 NULL;

	return dst_vma;
	}

	/*
	* 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;
	bool page_in_cache = page_mapping(page);
	spinlock_t *ptl;
	struct folio *folio;
	struct inode *inode;
	pgoff_t offset, max_off;

	_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);
	if (flags & MFILL_ATOMIC_WP)
	_dst_pte = pte_mkuffd_wp(_dst_pte);

	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);

	if (vma_is_shmem(dst_vma)) {
	/* serialize against truncate with the page table lock */
	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);
	ret = -EFAULT;
	if (unlikely(offset >= max_off))
	goto out_unlock;
	}

	ret = -EEXIST;
	/*
	* We allow to overwrite a 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_mostly(*dst_pte))
	goto out_unlock;

	folio = page_folio(page);
	if (page_in_cache) {
	/* Usually, cache pages are already added to LRU */
	if (newly_allocated)
	folio_add_lru(folio);
	page_add_file_rmap(page, dst_vma, false);
	} else {
	page_add_new_anon_rmap(page, dst_vma, dst_addr);
	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(page));

	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);
	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, false);
	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_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;
	pgoff_t offset, max_off;
	struct inode *inode;

	_dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
	dst_vma->vm_page_prot));
	dst_pte = pte_offset_map_lock(dst_vma->vm_mm, dst_pmd, dst_addr, &ptl);
	if (dst_vma->vm_file) {
	/* the shmem MAP_PRIVATE case requires checking the i_size */
	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);
	ret = -EFAULT;
	if (unlikely(offset >= max_off))
	goto out_unlock;
	}
	ret = -EEXIST;
	if (!pte_none(*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);
	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, &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;
	}

	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 mmap_lock held, it will release mmap_lock before returning.
	*/
	static __always_inline ssize_t mfill_atomic_hugetlb(
	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;
	int vm_shared = dst_vma->vm_flags & VM_SHARED;
	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)) {
	mmap_read_unlock(dst_mm);
	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) {
	err = -ENOENT;
	dst_vma = find_dst_vma(dst_mm, dst_start, len);
	if (!dst_vma \|\| !is_vm_hugetlb_page(dst_vma))
	goto out_unlock;

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

	vm_shared = dst_vma->vm_flags & VM_SHARED;
	}

	/*
	* If not shared, ensure the dst_vma has a anon_vma.
	*/
	err = -ENOMEM;
	if (!vm_shared) {
	if (unlikely(anon_vma_prepare(dst_vma)))
	goto out_unlock;
	}

	while (src_addr < src_start + len) {
	BUG_ON(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) &&
	!huge_pte_none_mostly(huge_ptep_get(dst_pte))) {
	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)) {
	mmap_read_unlock(dst_mm);
	BUG_ON(!folio);

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

	dst_vma = NULL;
	goto retry;
	} else
	BUG_ON(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:
	mmap_read_unlock(dst_mm);
	out:
	if (folio)
	folio_put(folio);
	BUG_ON(copied < 0);
	BUG_ON(err > 0);
	BUG_ON(!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 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);
	}

	/*
	* 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 mm_struct *dst_mm,
	unsigned long dst_start,
	unsigned long src_start,
	unsigned long len,
	atomic_t *mmap_changing,
	uffd_flags_t flags)
	{
	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:
	*/
	BUG_ON(dst_start & ~PAGE_MASK);
	BUG_ON(len & ~PAGE_MASK);

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

	src_addr = src_start;
	dst_addr = dst_start;
	copied = 0;
	folio = NULL;
	retry:
	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
	*/
	err = -EAGAIN;
	if (mmap_changing && atomic_read(mmap_changing))
	goto out_unlock;

	/*
	* Make sure the vma is not shared, that the dst range is
	* both valid and fully within a single existing vma.
	*/
	err = -ENOENT;
	dst_vma = find_dst_vma(dst_mm, dst_start, len);
	if (!dst_vma)
	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(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;

	/*
	* Ensure the dst_vma has a anon_vma or this page
	* would get a NULL anon_vma when moved in the
	* dst_vma.
	*/
	err = -ENOMEM;
	if (!(dst_vma->vm_flags & VM_SHARED) &&
	unlikely(anon_vma_prepare(dst_vma)))
	goto out_unlock;

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

	BUG_ON(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 the dst_pmd is mapped as THP don't
	* override it and just be strict.
	*/
	if (unlikely(pmd_trans_huge(dst_pmdval))) {
	err = -EEXIST;
	break;
	}
	if (unlikely(pmd_none(dst_pmdval)) &&
	unlikely(__pte_alloc(dst_mm, dst_pmd))) {
	err = -ENOMEM;
	break;
	}
	/* If an huge pmd materialized from under us fail */
	if (unlikely(pmd_trans_huge(*dst_pmd))) {
	err = -EFAULT;
	break;
	}

	BUG_ON(pmd_none(*dst_pmd));
	BUG_ON(pmd_trans_huge(*dst_pmd));

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

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

	mmap_read_unlock(dst_mm);
	BUG_ON(!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
	BUG_ON(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:
	mmap_read_unlock(dst_mm);
	out:
	if (folio)
	folio_put(folio);
	BUG_ON(copied < 0);
	BUG_ON(err > 0);
	BUG_ON(!copied && !err);
	return copied ? copied : err;
	}

	ssize_t mfill_atomic_copy(struct mm_struct *dst_mm, unsigned long dst_start,
	unsigned long src_start, unsigned long len,
	atomic_t *mmap_changing, uffd_flags_t flags)
	{
	return mfill_atomic(dst_mm, dst_start, src_start, len, mmap_changing,
	uffd_flags_set_mode(flags, MFILL_ATOMIC_COPY));
	}

	ssize_t mfill_atomic_zeropage(struct mm_struct *dst_mm, unsigned long start,
	unsigned long len, atomic_t *mmap_changing)
	{
	return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
	uffd_flags_set_mode(0, MFILL_ATOMIC_ZEROPAGE));
	}

	ssize_t mfill_atomic_continue(struct mm_struct *dst_mm, unsigned long start,
	unsigned long len, atomic_t *mmap_changing,
	uffd_flags_t flags)
	{
	return mfill_atomic(dst_mm, start, 0, len, mmap_changing,
	uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
	}

	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 mm_struct *dst_mm, unsigned long start,
	unsigned long len, bool enable_wp,
	atomic_t *mmap_changing)
	{
	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:
	*/
	BUG_ON(start & ~PAGE_MASK);
	BUG_ON(len & ~PAGE_MASK);

	/* Does the address range wrap, or is the span zero-sized? */
	BUG_ON(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
	*/
	err = -EAGAIN;
	if (mmap_changing && atomic_read(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:
	mmap_read_unlock(dst_mm);
	return err;
	}