#include <linux/kernel.h>
  #include <linux/errno.h>
  #include <linux/err.h>
  #include <linux/spinlock.h>

  #include <linux/mm.h>

  #include <linux/memremap.h>

  #include <linux/pagemap.h>
  #include <linux/rmap.h>
  #include <linux/swap.h>
  #include <linux/swapops.h>

  #include <linux/sched.h>
  #include <linux/rwsem.h>

  #include <linux/hugetlb.h>

  #include <asm/mmu_context.h>

  #include <asm/pgtable.h>

  #include <asm/tlbflush.h>

  #include "internal.h"

  static struct page *no_page_table(struct vm_area_struct *vma,
  		unsigned int flags)

  {

  	/*
  	 * When core dumping an enormous anonymous area that nobody
  	 * has touched so far, we don't want to allocate unnecessary pages or
  	 * page tables.  Return error instead of NULL to skip handle_mm_fault,
  	 * then get_dump_page() will return NULL to leave a hole in the dump.
  	 * But we can only make this optimization where a hole would surely
  	 * be zero-filled if handle_mm_fault() actually did handle it.
  	 */
  	if ((flags & FOLL_DUMP) && (!vma->vm_ops || !vma->vm_ops->fault))
  		return ERR_PTR(-EFAULT);
  	return NULL;
  }

  static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
  		pte_t *pte, unsigned int flags)
  {
  	/* No page to get reference */
  	if (flags & FOLL_GET)
  		return -EFAULT;
  
  	if (flags & FOLL_TOUCH) {
  		pte_t entry = *pte;
  
  		if (flags & FOLL_WRITE)
  			entry = pte_mkdirty(entry);
  		entry = pte_mkyoung(entry);
  
  		if (!pte_same(*pte, entry)) {
  			set_pte_at(vma->vm_mm, address, pte, entry);
  			update_mmu_cache(vma, address, pte);
  		}
  	}
  
  	/* Proper page table entry exists, but no corresponding struct page */
  	return -EEXIST;
  }

  /*
   * FOLL_FORCE can write to even unwritable pte's, but only
   * after we've gone through a COW cycle and they are dirty.
   */
  static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
  {
  	return pte_write(pte) ||
  		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
  }

  static struct page *follow_page_pte(struct vm_area_struct *vma,
  		unsigned long address, pmd_t *pmd, unsigned int flags)
  {
  	struct mm_struct *mm = vma->vm_mm;

  	struct dev_pagemap *pgmap = NULL;

  	struct page *page;
  	spinlock_t *ptl;
  	pte_t *ptep, pte;

  retry:

  	if (unlikely(pmd_bad(*pmd)))

  		return no_page_table(vma, flags);

  
  	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);

  	pte = *ptep;
  	if (!pte_present(pte)) {
  		swp_entry_t entry;
  		/*
  		 * KSM's break_ksm() relies upon recognizing a ksm page
  		 * even while it is being migrated, so for that case we
  		 * need migration_entry_wait().
  		 */
  		if (likely(!(flags & FOLL_MIGRATION)))
  			goto no_page;

  		if (pte_none(pte))

  			goto no_page;
  		entry = pte_to_swp_entry(pte);
  		if (!is_migration_entry(entry))
  			goto no_page;
  		pte_unmap_unlock(ptep, ptl);
  		migration_entry_wait(mm, pmd, address);

  		goto retry;

  	}

  	if ((flags & FOLL_NUMA) && pte_protnone(pte))

  		goto no_page;

  	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {

  		pte_unmap_unlock(ptep, ptl);
  		return NULL;
  	}

  
  	page = vm_normal_page(vma, address, pte);

  	if (!page && pte_devmap(pte) && (flags & FOLL_GET)) {
  		/*
  		 * Only return device mapping pages in the FOLL_GET case since
  		 * they are only valid while holding the pgmap reference.
  		 */
  		pgmap = get_dev_pagemap(pte_pfn(pte), NULL);
  		if (pgmap)
  			page = pte_page(pte);
  		else
  			goto no_page;
  	} else if (unlikely(!page)) {

  		if (flags & FOLL_DUMP) {
  			/* Avoid special (like zero) pages in core dumps */
  			page = ERR_PTR(-EFAULT);
  			goto out;
  		}
  
  		if (is_zero_pfn(pte_pfn(pte))) {
  			page = pte_page(pte);
  		} else {
  			int ret;
  
  			ret = follow_pfn_pte(vma, address, ptep, flags);
  			page = ERR_PTR(ret);
  			goto out;
  		}

  	}

  	if (flags & FOLL_SPLIT && PageTransCompound(page)) {
  		int ret;
  		get_page(page);
  		pte_unmap_unlock(ptep, ptl);
  		lock_page(page);
  		ret = split_huge_page(page);
  		unlock_page(page);
  		put_page(page);
  		if (ret)
  			return ERR_PTR(ret);
  		goto retry;
  	}

  	if (flags & FOLL_GET) {

  		get_page(page);

  
  		/* drop the pgmap reference now that we hold the page */
  		if (pgmap) {
  			put_dev_pagemap(pgmap);
  			pgmap = NULL;
  		}
  	}

  	if (flags & FOLL_TOUCH) {
  		if ((flags & FOLL_WRITE) &&
  		    !pte_dirty(pte) && !PageDirty(page))
  			set_page_dirty(page);
  		/*
  		 * pte_mkyoung() would be more correct here, but atomic care
  		 * is needed to avoid losing the dirty bit: it is easier to use
  		 * mark_page_accessed().
  		 */
  		mark_page_accessed(page);
  	}

  	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {

  		/* Do not mlock pte-mapped THP */
  		if (PageTransCompound(page))
  			goto out;

  		/*
  		 * The preliminary mapping check is mainly to avoid the
  		 * pointless overhead of lock_page on the ZERO_PAGE
  		 * which might bounce very badly if there is contention.
  		 *
  		 * If the page is already locked, we don't need to
  		 * handle it now - vmscan will handle it later if and
  		 * when it attempts to reclaim the page.
  		 */
  		if (page->mapping && trylock_page(page)) {
  			lru_add_drain();  /* push cached pages to LRU */
  			/*
  			 * Because we lock page here, and migration is
  			 * blocked by the pte's page reference, and we
  			 * know the page is still mapped, we don't even
  			 * need to check for file-cache page truncation.
  			 */
  			mlock_vma_page(page);
  			unlock_page(page);
  		}
  	}

  out:

  	pte_unmap_unlock(ptep, ptl);

  	return page;

  no_page:
  	pte_unmap_unlock(ptep, ptl);
  	if (!pte_none(pte))

  		return NULL;
  	return no_page_table(vma, flags);
  }
  
  /**
   * follow_page_mask - look up a page descriptor from a user-virtual address
   * @vma: vm_area_struct mapping @address
   * @address: virtual address to look up
   * @flags: flags modifying lookup behaviour
   * @page_mask: on output, *page_mask is set according to the size of the page
   *
   * @flags can have FOLL_ flags set, defined in <linux/mm.h>
   *
   * Returns the mapped (struct page *), %NULL if no mapping exists, or
   * an error pointer if there is a mapping to something not represented
   * by a page descriptor (see also vm_normal_page()).
   */
  struct page *follow_page_mask(struct vm_area_struct *vma,
  			      unsigned long address, unsigned int flags,
  			      unsigned int *page_mask)
  {
  	pgd_t *pgd;
  	pud_t *pud;
  	pmd_t *pmd;
  	spinlock_t *ptl;
  	struct page *page;
  	struct mm_struct *mm = vma->vm_mm;
  
  	*page_mask = 0;
  
  	page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
  	if (!IS_ERR(page)) {
  		BUG_ON(flags & FOLL_GET);

  		return page;

  	}

  	pgd = pgd_offset(mm, address);
  	if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
  		return no_page_table(vma, flags);
  
  	pud = pud_offset(pgd, address);
  	if (pud_none(*pud))
  		return no_page_table(vma, flags);
  	if (pud_huge(*pud) && vma->vm_flags & VM_HUGETLB) {

  		page = follow_huge_pud(mm, address, pud, flags);
  		if (page)
  			return page;
  		return no_page_table(vma, flags);

  	}
  	if (unlikely(pud_bad(*pud)))
  		return no_page_table(vma, flags);
  
  	pmd = pmd_offset(pud, address);
  	if (pmd_none(*pmd))
  		return no_page_table(vma, flags);
  	if (pmd_huge(*pmd) && vma->vm_flags & VM_HUGETLB) {

  		page = follow_huge_pmd(mm, address, pmd, flags);
  		if (page)
  			return page;
  		return no_page_table(vma, flags);

  	}

  	if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))

  		return no_page_table(vma, flags);

  	if (pmd_devmap(*pmd)) {
  		ptl = pmd_lock(mm, pmd);
  		page = follow_devmap_pmd(vma, address, pmd, flags);
  		spin_unlock(ptl);
  		if (page)
  			return page;
  	}

  	if (likely(!pmd_trans_huge(*pmd)))
  		return follow_page_pte(vma, address, pmd, flags);
  
  	ptl = pmd_lock(mm, pmd);
  	if (unlikely(!pmd_trans_huge(*pmd))) {
  		spin_unlock(ptl);
  		return follow_page_pte(vma, address, pmd, flags);
  	}

  	if (flags & FOLL_SPLIT) {
  		int ret;
  		page = pmd_page(*pmd);
  		if (is_huge_zero_page(page)) {
  			spin_unlock(ptl);
  			ret = 0;

  			split_huge_pmd(vma, pmd, address);

  			if (pmd_trans_unstable(pmd))
  				ret = -EBUSY;

  		} else {
  			get_page(page);

  			spin_unlock(ptl);

  			lock_page(page);
  			ret = split_huge_page(page);
  			unlock_page(page);
  			put_page(page);

  			if (pmd_none(*pmd))
  				return no_page_table(vma, flags);

  		}
  
  		return ret ? ERR_PTR(ret) :
  			follow_page_pte(vma, address, pmd, flags);

  	}

  
  	page = follow_trans_huge_pmd(vma, address, pmd, flags);
  	spin_unlock(ptl);
  	*page_mask = HPAGE_PMD_NR - 1;
  	return page;

  }

  static int get_gate_page(struct mm_struct *mm, unsigned long address,
  		unsigned int gup_flags, struct vm_area_struct **vma,
  		struct page **page)
  {
  	pgd_t *pgd;
  	pud_t *pud;
  	pmd_t *pmd;
  	pte_t *pte;
  	int ret = -EFAULT;
  
  	/* user gate pages are read-only */
  	if (gup_flags & FOLL_WRITE)
  		return -EFAULT;
  	if (address > TASK_SIZE)
  		pgd = pgd_offset_k(address);
  	else
  		pgd = pgd_offset_gate(mm, address);
  	BUG_ON(pgd_none(*pgd));
  	pud = pud_offset(pgd, address);
  	BUG_ON(pud_none(*pud));
  	pmd = pmd_offset(pud, address);
  	if (pmd_none(*pmd))
  		return -EFAULT;
  	VM_BUG_ON(pmd_trans_huge(*pmd));
  	pte = pte_offset_map(pmd, address);
  	if (pte_none(*pte))
  		goto unmap;
  	*vma = get_gate_vma(mm);
  	if (!page)
  		goto out;
  	*page = vm_normal_page(*vma, address, *pte);
  	if (!*page) {
  		if ((gup_flags & FOLL_DUMP) || !is_zero_pfn(pte_pfn(*pte)))
  			goto unmap;
  		*page = pte_page(*pte);
  	}
  	get_page(*page);
  out:
  	ret = 0;
  unmap:
  	pte_unmap(pte);
  	return ret;
  }

  /*
   * mmap_sem must be held on entry.  If @nonblocking != NULL and
   * *@flags does not include FOLL_NOWAIT, the mmap_sem may be released.
   * If it is, *@nonblocking will be set to 0 and -EBUSY returned.
   */

  static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
  		unsigned long address, unsigned int *flags, int *nonblocking)
  {

  	unsigned int fault_flags = 0;
  	int ret;

  	/* mlock all present pages, but do not fault in new pages */
  	if ((*flags & (FOLL_POPULATE | FOLL_MLOCK)) == FOLL_MLOCK)
  		return -ENOENT;

  	/* For mm_populate(), just skip the stack guard page. */
  	if ((*flags & FOLL_POPULATE) &&

  			(stack_guard_page_start(vma, address) ||
  			 stack_guard_page_end(vma, address + PAGE_SIZE)))
  		return -ENOENT;
  	if (*flags & FOLL_WRITE)
  		fault_flags |= FAULT_FLAG_WRITE;

  	if (*flags & FOLL_REMOTE)
  		fault_flags |= FAULT_FLAG_REMOTE;

  	if (nonblocking)
  		fault_flags |= FAULT_FLAG_ALLOW_RETRY;
  	if (*flags & FOLL_NOWAIT)
  		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;

  	if (*flags & FOLL_TRIED) {
  		VM_WARN_ON_ONCE(fault_flags & FAULT_FLAG_ALLOW_RETRY);
  		fault_flags |= FAULT_FLAG_TRIED;
  	}

  	ret = handle_mm_fault(vma, address, fault_flags);

  	if (ret & VM_FAULT_ERROR) {
  		if (ret & VM_FAULT_OOM)
  			return -ENOMEM;
  		if (ret & (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE))
  			return *flags & FOLL_HWPOISON ? -EHWPOISON : -EFAULT;

  		if (ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV))

  			return -EFAULT;
  		BUG();
  	}
  
  	if (tsk) {
  		if (ret & VM_FAULT_MAJOR)
  			tsk->maj_flt++;
  		else
  			tsk->min_flt++;
  	}
  
  	if (ret & VM_FAULT_RETRY) {
  		if (nonblocking)
  			*nonblocking = 0;
  		return -EBUSY;
  	}
  
  	/*
  	 * The VM_FAULT_WRITE bit tells us that do_wp_page has broken COW when
  	 * necessary, even if maybe_mkwrite decided not to set pte_write. We
  	 * can thus safely do subsequent page lookups as if they were reads.
  	 * But only do so when looping for pte_write is futile: in some cases
  	 * userspace may also be wanting to write to the gotten user page,
  	 * which a read fault here might prevent (a readonly page might get
  	 * reCOWed by userspace write).
  	 */
  	if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))

  	        *flags |= FOLL_COW;

  	return 0;
  }

  static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
  {
  	vm_flags_t vm_flags = vma->vm_flags;

  	int write = (gup_flags & FOLL_WRITE);
  	int foreign = (gup_flags & FOLL_REMOTE);

  
  	if (vm_flags & (VM_IO | VM_PFNMAP))
  		return -EFAULT;

  	if (write) {

  		if (!(vm_flags & VM_WRITE)) {
  			if (!(gup_flags & FOLL_FORCE))
  				return -EFAULT;
  			/*
  			 * We used to let the write,force case do COW in a
  			 * VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could
  			 * set a breakpoint in a read-only mapping of an
  			 * executable, without corrupting the file (yet only
  			 * when that file had been opened for writing!).
  			 * Anon pages in shared mappings are surprising: now
  			 * just reject it.
  			 */

  			if (!is_cow_mapping(vm_flags))

  				return -EFAULT;

  		}
  	} else if (!(vm_flags & VM_READ)) {
  		if (!(gup_flags & FOLL_FORCE))
  			return -EFAULT;
  		/*
  		 * Is there actually any vma we can reach here which does not
  		 * have VM_MAYREAD set?
  		 */
  		if (!(vm_flags & VM_MAYREAD))
  			return -EFAULT;
  	}

  	/*
  	 * gups are always data accesses, not instruction
  	 * fetches, so execute=false here
  	 */
  	if (!arch_vma_access_permitted(vma, write, false, foreign))

  		return -EFAULT;

  	return 0;
  }

  /**
   * __get_user_pages() - pin user pages in memory
   * @tsk:	task_struct of target task
   * @mm:		mm_struct of target mm
   * @start:	starting user address
   * @nr_pages:	number of pages from start to pin
   * @gup_flags:	flags modifying pin behaviour
   * @pages:	array that receives pointers to the pages pinned.
   *		Should be at least nr_pages long. Or NULL, if caller
   *		only intends to ensure the pages are faulted in.
   * @vmas:	array of pointers to vmas corresponding to each page.
   *		Or NULL if the caller does not require them.
   * @nonblocking: whether waiting for disk IO or mmap_sem contention
   *
   * Returns number of pages pinned. This may be fewer than the number
   * requested. If nr_pages is 0 or negative, returns 0. If no pages
   * were pinned, returns -errno. Each page returned must be released
   * with a put_page() call when it is finished with. vmas will only
   * remain valid while mmap_sem is held.
   *

   * Must be called with mmap_sem held.  It may be released.  See below.

   *
   * __get_user_pages walks a process's page tables and takes a reference to
   * each struct page that each user address corresponds to at a given
   * instant. That is, it takes the page that would be accessed if a user
   * thread accesses the given user virtual address at that instant.
   *
   * This does not guarantee that the page exists in the user mappings when
   * __get_user_pages returns, and there may even be a completely different
   * page there in some cases (eg. if mmapped pagecache has been invalidated
   * and subsequently re faulted). However it does guarantee that the page
   * won't be freed completely. And mostly callers simply care that the page
   * contains data that was valid *at some point in time*. Typically, an IO
   * or similar operation cannot guarantee anything stronger anyway because
   * locks can't be held over the syscall boundary.
   *
   * If @gup_flags & FOLL_WRITE == 0, the page must not be written to. If
   * the page is written to, set_page_dirty (or set_page_dirty_lock, as
   * appropriate) must be called after the page is finished with, and
   * before put_page is called.
   *
   * If @nonblocking != NULL, __get_user_pages will not wait for disk IO
   * or mmap_sem contention, and if waiting is needed to pin all pages,

   * *@nonblocking will be set to 0.  Further, if @gup_flags does not
   * include FOLL_NOWAIT, the mmap_sem will be released via up_read() in
   * this case.
   *
   * A caller using such a combination of @nonblocking and @gup_flags
   * must therefore hold the mmap_sem for reading only, and recognize
   * when it's been released.  Otherwise, it must be held for either
   * reading or writing and will not be released.

   *
   * In most cases, get_user_pages or get_user_pages_fast should be used
   * instead of __get_user_pages. __get_user_pages should be used only if
   * you need some special @gup_flags.
   */

  static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,

  		unsigned long start, unsigned long nr_pages,
  		unsigned int gup_flags, struct page **pages,
  		struct vm_area_struct **vmas, int *nonblocking)
  {

  	long i = 0;

  	unsigned int page_mask;

  	struct vm_area_struct *vma = NULL;

  
  	if (!nr_pages)
  		return 0;
  
  	VM_BUG_ON(!!pages != !!(gup_flags & FOLL_GET));
  
  	/*
  	 * If FOLL_FORCE is set then do not force a full fault as the hinting
  	 * fault information is unrelated to the reference behaviour of a task
  	 * using the address space
  	 */
  	if (!(gup_flags & FOLL_FORCE))
  		gup_flags |= FOLL_NUMA;

  	do {

  		struct page *page;
  		unsigned int foll_flags = gup_flags;
  		unsigned int page_increm;
  
  		/* first iteration or cross vma bound */
  		if (!vma || start >= vma->vm_end) {
  			vma = find_extend_vma(mm, start);
  			if (!vma && in_gate_area(mm, start)) {
  				int ret;
  				ret = get_gate_page(mm, start & PAGE_MASK,
  						gup_flags, &vma,
  						pages ? &pages[i] : NULL);
  				if (ret)
  					return i ? : ret;
  				page_mask = 0;
  				goto next_page;
  			}

  			if (!vma || check_vma_flags(vma, gup_flags))
  				return i ? : -EFAULT;
  			if (is_vm_hugetlb_page(vma)) {
  				i = follow_hugetlb_page(mm, vma, pages, vmas,
  						&start, &nr_pages, i,
  						gup_flags);
  				continue;

  			}

  		}
  retry:
  		/*
  		 * If we have a pending SIGKILL, don't keep faulting pages and
  		 * potentially allocating memory.
  		 */
  		if (unlikely(fatal_signal_pending(current)))
  			return i ? i : -ERESTARTSYS;
  		cond_resched();
  		page = follow_page_mask(vma, start, foll_flags, &page_mask);
  		if (!page) {
  			int ret;
  			ret = faultin_page(tsk, vma, start, &foll_flags,
  					nonblocking);
  			switch (ret) {
  			case 0:
  				goto retry;
  			case -EFAULT:
  			case -ENOMEM:
  			case -EHWPOISON:
  				return i ? i : ret;
  			case -EBUSY:
  				return i;
  			case -ENOENT:
  				goto next_page;

  			}

  			BUG();

  		} else if (PTR_ERR(page) == -EEXIST) {
  			/*
  			 * Proper page table entry exists, but no corresponding
  			 * struct page.
  			 */
  			goto next_page;
  		} else if (IS_ERR(page)) {

  			return i ? i : PTR_ERR(page);

  		}

  		if (pages) {
  			pages[i] = page;
  			flush_anon_page(vma, page, start);
  			flush_dcache_page(page);
  			page_mask = 0;

  		}

  next_page:

  		if (vmas) {
  			vmas[i] = vma;
  			page_mask = 0;
  		}
  		page_increm = 1 + (~(start >> PAGE_SHIFT) & page_mask);
  		if (page_increm > nr_pages)
  			page_increm = nr_pages;
  		i += page_increm;
  		start += page_increm * PAGE_SIZE;
  		nr_pages -= page_increm;

  	} while (nr_pages);
  	return i;

  }

  bool vma_permits_fault(struct vm_area_struct *vma, unsigned int fault_flags)
  {

  	bool write   = !!(fault_flags & FAULT_FLAG_WRITE);
  	bool foreign = !!(fault_flags & FAULT_FLAG_REMOTE);

  	vm_flags_t vm_flags = write ? VM_WRITE : VM_READ;

  
  	if (!(vm_flags & vma->vm_flags))
  		return false;

  	/*
  	 * The architecture might have a hardware protection

  	 * mechanism other than read/write that can deny access.

  	 *
  	 * gup always represents data access, not instruction
  	 * fetches, so execute=false here:

  	 */

  	if (!arch_vma_access_permitted(vma, write, false, foreign))

  		return false;

  	return true;
  }

  /*
   * fixup_user_fault() - manually resolve a user page fault
   * @tsk:	the task_struct to use for page fault accounting, or
   *		NULL if faults are not to be recorded.
   * @mm:		mm_struct of target mm
   * @address:	user address
   * @fault_flags:flags to pass down to handle_mm_fault()

   * @unlocked:	did we unlock the mmap_sem while retrying, maybe NULL if caller
   *		does not allow retry

   *
   * This is meant to be called in the specific scenario where for locking reasons
   * we try to access user memory in atomic context (within a pagefault_disable()
   * section), this returns -EFAULT, and we want to resolve the user fault before
   * trying again.
   *
   * Typically this is meant to be used by the futex code.
   *
   * The main difference with get_user_pages() is that this function will
   * unconditionally call handle_mm_fault() which will in turn perform all the
   * necessary SW fixup of the dirty and young bits in the PTE, while

   * get_user_pages() only guarantees to update these in the struct page.

   *
   * This is important for some architectures where those bits also gate the
   * access permission to the page because they are maintained in software.  On
   * such architectures, gup() will not be enough to make a subsequent access
   * succeed.
   *

   * This function will not return with an unlocked mmap_sem. So it has not the
   * same semantics wrt the @mm->mmap_sem as does filemap_fault().

   */
  int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,

  		     unsigned long address, unsigned int fault_flags,
  		     bool *unlocked)

  {
  	struct vm_area_struct *vma;

  	int ret, major = 0;
  
  	if (unlocked)
  		fault_flags |= FAULT_FLAG_ALLOW_RETRY;

  retry:

  	vma = find_extend_vma(mm, address);
  	if (!vma || address < vma->vm_start)
  		return -EFAULT;

  	if (!vma_permits_fault(vma, fault_flags))

  		return -EFAULT;

  	ret = handle_mm_fault(vma, address, fault_flags);

  	major |= ret & VM_FAULT_MAJOR;

  	if (ret & VM_FAULT_ERROR) {
  		if (ret & VM_FAULT_OOM)
  			return -ENOMEM;
  		if (ret & (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE))
  			return -EHWPOISON;

  		if (ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV))

  			return -EFAULT;
  		BUG();
  	}

  
  	if (ret & VM_FAULT_RETRY) {
  		down_read(&mm->mmap_sem);
  		if (!(fault_flags & FAULT_FLAG_TRIED)) {
  			*unlocked = true;
  			fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
  			fault_flags |= FAULT_FLAG_TRIED;
  			goto retry;
  		}
  	}

  	if (tsk) {

  		if (major)

  			tsk->maj_flt++;
  		else
  			tsk->min_flt++;
  	}
  	return 0;
  }

  EXPORT_SYMBOL_GPL(fixup_user_fault);

  static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
  						struct mm_struct *mm,
  						unsigned long start,
  						unsigned long nr_pages,

  						struct page **pages,
  						struct vm_area_struct **vmas,

  						int *locked, bool notify_drop,
  						unsigned int flags)

  {

  	long ret, pages_done;
  	bool lock_dropped;
  
  	if (locked) {
  		/* if VM_FAULT_RETRY can be returned, vmas become invalid */
  		BUG_ON(vmas);
  		/* check caller initialized locked */
  		BUG_ON(*locked != 1);
  	}
  
  	if (pages)
  		flags |= FOLL_GET;

  
  	pages_done = 0;
  	lock_dropped = false;
  	for (;;) {
  		ret = __get_user_pages(tsk, mm, start, nr_pages, flags, pages,
  				       vmas, locked);
  		if (!locked)
  			/* VM_FAULT_RETRY couldn't trigger, bypass */
  			return ret;
  
  		/* VM_FAULT_RETRY cannot return errors */
  		if (!*locked) {
  			BUG_ON(ret < 0);
  			BUG_ON(ret >= nr_pages);
  		}
  
  		if (!pages)
  			/* If it's a prefault don't insist harder */
  			return ret;
  
  		if (ret > 0) {
  			nr_pages -= ret;
  			pages_done += ret;
  			if (!nr_pages)
  				break;
  		}
  		if (*locked) {
  			/* VM_FAULT_RETRY didn't trigger */
  			if (!pages_done)
  				pages_done = ret;
  			break;
  		}
  		/* VM_FAULT_RETRY triggered, so seek to the faulting offset */
  		pages += ret;
  		start += ret << PAGE_SHIFT;
  
  		/*
  		 * Repeat on the address that fired VM_FAULT_RETRY
  		 * without FAULT_FLAG_ALLOW_RETRY but with
  		 * FAULT_FLAG_TRIED.
  		 */
  		*locked = 1;
  		lock_dropped = true;
  		down_read(&mm->mmap_sem);
  		ret = __get_user_pages(tsk, mm, start, 1, flags | FOLL_TRIED,
  				       pages, NULL, NULL);
  		if (ret != 1) {
  			BUG_ON(ret > 1);
  			if (!pages_done)
  				pages_done = ret;
  			break;
  		}
  		nr_pages--;
  		pages_done++;
  		if (!nr_pages)
  			break;
  		pages++;
  		start += PAGE_SIZE;
  	}
  	if (notify_drop && lock_dropped && *locked) {
  		/*
  		 * We must let the caller know we temporarily dropped the lock
  		 * and so the critical section protected by it was lost.
  		 */
  		up_read(&mm->mmap_sem);
  		*locked = 0;
  	}
  	return pages_done;
  }
  
  /*
   * We can leverage the VM_FAULT_RETRY functionality in the page fault
   * paths better by using either get_user_pages_locked() or
   * get_user_pages_unlocked().
   *
   * get_user_pages_locked() is suitable to replace the form:
   *
   *      down_read(&mm->mmap_sem);
   *      do_something()
   *      get_user_pages(tsk, mm, ..., pages, NULL);
   *      up_read(&mm->mmap_sem);
   *
   *  to:
   *
   *      int locked = 1;
   *      down_read(&mm->mmap_sem);
   *      do_something()
   *      get_user_pages_locked(tsk, mm, ..., pages, &locked);
   *      if (locked)
   *          up_read(&mm->mmap_sem);
   */

  long get_user_pages_locked(unsigned long start, unsigned long nr_pages,

  			   unsigned int gup_flags, struct page **pages,

  			   int *locked)
  {

  	return __get_user_pages_locked(current, current->mm, start, nr_pages,

  				       pages, NULL, locked, true,
  				       gup_flags | FOLL_TOUCH);

  }

  EXPORT_SYMBOL(get_user_pages_locked);

  
  /*

   * Same as get_user_pages_unlocked(...., FOLL_TOUCH) but it allows to
   * pass additional gup_flags as last parameter (like FOLL_HWPOISON).
   *
   * NOTE: here FOLL_TOUCH is not set implicitly and must be set by the
   * caller if required (just like with __get_user_pages). "FOLL_GET",
   * "FOLL_WRITE" and "FOLL_FORCE" are set implicitly as needed
   * according to the parameters "pages", "write", "force"
   * respectively.
   */
  __always_inline long __get_user_pages_unlocked(struct task_struct *tsk, struct mm_struct *mm,
  					       unsigned long start, unsigned long nr_pages,

  					       struct page **pages, unsigned int gup_flags)

  {
  	long ret;
  	int locked = 1;

  	down_read(&mm->mmap_sem);

  	ret = __get_user_pages_locked(tsk, mm, start, nr_pages, pages, NULL,
  				      &locked, false, gup_flags);

  	if (locked)
  		up_read(&mm->mmap_sem);
  	return ret;
  }
  EXPORT_SYMBOL(__get_user_pages_unlocked);
  
  /*

   * get_user_pages_unlocked() is suitable to replace the form:
   *
   *      down_read(&mm->mmap_sem);
   *      get_user_pages(tsk, mm, ..., pages, NULL);
   *      up_read(&mm->mmap_sem);
   *
   *  with:
   *
   *      get_user_pages_unlocked(tsk, mm, ..., pages);
   *
   * It is functionally equivalent to get_user_pages_fast so
   * get_user_pages_fast should be used instead, if the two parameters
   * "tsk" and "mm" are respectively equal to current and current->mm,
   * or if "force" shall be set to 1 (get_user_pages_fast misses the
   * "force" parameter).
   */

  long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,

  			     struct page **pages, unsigned int gup_flags)

  {

  	return __get_user_pages_unlocked(current, current->mm, start, nr_pages,

  					 pages, gup_flags | FOLL_TOUCH);

  }

  EXPORT_SYMBOL(get_user_pages_unlocked);

  /*

   * get_user_pages_remote() - pin user pages in memory

   * @tsk:	the task_struct to use for page fault accounting, or
   *		NULL if faults are not to be recorded.
   * @mm:		mm_struct of target mm
   * @start:	starting user address
   * @nr_pages:	number of pages from start to pin

   * @gup_flags:	flags modifying lookup behaviour

   * @pages:	array that receives pointers to the pages pinned.
   *		Should be at least nr_pages long. Or NULL, if caller
   *		only intends to ensure the pages are faulted in.
   * @vmas:	array of pointers to vmas corresponding to each page.
   *		Or NULL if the caller does not require them.
   *
   * Returns number of pages pinned. This may be fewer than the number
   * requested. If nr_pages is 0 or negative, returns 0. If no pages
   * were pinned, returns -errno. Each page returned must be released
   * with a put_page() call when it is finished with. vmas will only
   * remain valid while mmap_sem is held.
   *
   * Must be called with mmap_sem held for read or write.
   *
   * get_user_pages walks a process's page tables and takes a reference to
   * each struct page that each user address corresponds to at a given
   * instant. That is, it takes the page that would be accessed if a user
   * thread accesses the given user virtual address at that instant.
   *
   * This does not guarantee that the page exists in the user mappings when
   * get_user_pages returns, and there may even be a completely different
   * page there in some cases (eg. if mmapped pagecache has been invalidated
   * and subsequently re faulted). However it does guarantee that the page
   * won't be freed completely. And mostly callers simply care that the page
   * contains data that was valid *at some point in time*. Typically, an IO
   * or similar operation cannot guarantee anything stronger anyway because
   * locks can't be held over the syscall boundary.
   *

   * If gup_flags & FOLL_WRITE == 0, the page must not be written to. If the page
   * is written to, set_page_dirty (or set_page_dirty_lock, as appropriate) must
   * be called after the page is finished with, and before put_page is called.

   *
   * get_user_pages is typically used for fewer-copy IO operations, to get a
   * handle on the memory by some means other than accesses via the user virtual
   * addresses. The pages may be submitted for DMA to devices or accessed via
   * their kernel linear mapping (via the kmap APIs). Care should be taken to
   * use the correct cache flushing APIs.
   *
   * See also get_user_pages_fast, for performance critical applications.

   *
   * get_user_pages should be phased out in favor of
   * get_user_pages_locked|unlocked or get_user_pages_fast. Nothing
   * should use get_user_pages because it cannot pass
   * FAULT_FLAG_ALLOW_RETRY to handle_mm_fault.

   */

  long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
  		unsigned long start, unsigned long nr_pages,

  		unsigned int gup_flags, struct page **pages,

  		struct vm_area_struct **vmas)

  {

  	return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,

  				       NULL, false,
  				       gup_flags | FOLL_TOUCH | FOLL_REMOTE);

  }
  EXPORT_SYMBOL(get_user_pages_remote);
  
  /*

   * This is the same as get_user_pages_remote(), just with a
   * less-flexible calling convention where we assume that the task
   * and mm being operated on are the current task's.  We also
   * obviously don't pass FOLL_REMOTE in here.

   */

  long get_user_pages(unsigned long start, unsigned long nr_pages,

  		unsigned int gup_flags, struct page **pages,

  		struct vm_area_struct **vmas)
  {

  	return __get_user_pages_locked(current, current->mm, start, nr_pages,

  				       pages, vmas, NULL, false,
  				       gup_flags | FOLL_TOUCH);

  }

  EXPORT_SYMBOL(get_user_pages);

  
  /**

   * populate_vma_page_range() -  populate a range of pages in the vma.
   * @vma:   target vma
   * @start: start address
   * @end:   end address
   * @nonblocking:
   *
   * This takes care of mlocking the pages too if VM_LOCKED is set.
   *
   * return 0 on success, negative error code on error.
   *
   * vma->vm_mm->mmap_sem must be held.
   *
   * If @nonblocking is NULL, it may be held for read or write and will
   * be unperturbed.
   *
   * If @nonblocking is non-NULL, it must held for read only and may be
   * released.  If it's released, *@nonblocking will be set to 0.
   */
  long populate_vma_page_range(struct vm_area_struct *vma,
  		unsigned long start, unsigned long end, int *nonblocking)
  {
  	struct mm_struct *mm = vma->vm_mm;
  	unsigned long nr_pages = (end - start) / PAGE_SIZE;
  	int gup_flags;
  
  	VM_BUG_ON(start & ~PAGE_MASK);
  	VM_BUG_ON(end   & ~PAGE_MASK);
  	VM_BUG_ON_VMA(start < vma->vm_start, vma);
  	VM_BUG_ON_VMA(end   > vma->vm_end, vma);
  	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);

  	gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK;
  	if (vma->vm_flags & VM_LOCKONFAULT)
  		gup_flags &= ~FOLL_POPULATE;

  	/*
  	 * We want to touch writable mappings with a write fault in order
  	 * to break COW, except for shared mappings because these don't COW
  	 * and we would not want to dirty them for nothing.
  	 */
  	if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE)
  		gup_flags |= FOLL_WRITE;
  
  	/*
  	 * We want mlock to succeed for regions that have any permissions
  	 * other than PROT_NONE.
  	 */
  	if (vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC))
  		gup_flags |= FOLL_FORCE;
  
  	/*
  	 * We made sure addr is within a VMA, so the following will
  	 * not result in a stack expansion that recurses back here.
  	 */
  	return __get_user_pages(current, mm, start, nr_pages, gup_flags,
  				NULL, NULL, nonblocking);
  }
  
  /*
   * __mm_populate - populate and/or mlock pages within a range of address space.
   *
   * This is used to implement mlock() and the MAP_POPULATE / MAP_LOCKED mmap
   * flags. VMAs must be already marked with the desired vm_flags, and
   * mmap_sem must not be held.
   */
  int __mm_populate(unsigned long start, unsigned long len, int ignore_errors)
  {
  	struct mm_struct *mm = current->mm;
  	unsigned long end, nstart, nend;
  	struct vm_area_struct *vma = NULL;
  	int locked = 0;
  	long ret = 0;
  
  	VM_BUG_ON(start & ~PAGE_MASK);
  	VM_BUG_ON(len != PAGE_ALIGN(len));
  	end = start + len;
  
  	for (nstart = start; nstart < end; nstart = nend) {
  		/*
  		 * We want to fault in pages for [nstart; end) address range.
  		 * Find first corresponding VMA.
  		 */
  		if (!locked) {
  			locked = 1;
  			down_read(&mm->mmap_sem);
  			vma = find_vma(mm, nstart);
  		} else if (nstart >= vma->vm_end)
  			vma = vma->vm_next;
  		if (!vma || vma->vm_start >= end)
  			break;
  		/*
  		 * Set [nstart; nend) to intersection of desired address
  		 * range with the first VMA. Also, skip undesirable VMA types.
  		 */
  		nend = min(end, vma->vm_end);
  		if (vma->vm_flags & (VM_IO | VM_PFNMAP))
  			continue;
  		if (nstart < vma->vm_start)
  			nstart = vma->vm_start;
  		/*
  		 * Now fault in a range of pages. populate_vma_page_range()
  		 * double checks the vma flags, so that it won't mlock pages
  		 * if the vma was already munlocked.
  		 */
  		ret = populate_vma_page_range(vma, nstart, nend, &locked);
  		if (ret < 0) {
  			if (ignore_errors) {
  				ret = 0;
  				continue;	/* continue at next VMA */
  			}
  			break;
  		}
  		nend = nstart + ret * PAGE_SIZE;
  		ret = 0;
  	}
  	if (locked)
  		up_read(&mm->mmap_sem);
  	return ret;	/* 0 or negative error code */
  }
  
  /**

   * get_dump_page() - pin user page in memory while writing it to core dump
   * @addr: user address
   *
   * Returns struct page pointer of user page pinned for dump,

   * to be freed afterwards by put_page().

   *
   * Returns NULL on any kind of failure - a hole must then be inserted into
   * the corefile, to preserve alignment with its headers; and also returns
   * NULL wherever the ZERO_PAGE, or an anonymous pte_none, has been found -
   * allowing a hole to be left in the corefile to save diskspace.
   *
   * Called without mmap_sem, but after all other threads have been killed.
   */
  #ifdef CONFIG_ELF_CORE
  struct page *get_dump_page(unsigned long addr)
  {
  	struct vm_area_struct *vma;
  	struct page *page;
  
  	if (__get_user_pages(current, current->mm, addr, 1,
  			     FOLL_FORCE | FOLL_DUMP | FOLL_GET, &page, &vma,
  			     NULL) < 1)
  		return NULL;
  	flush_cache_page(vma, addr, page_to_pfn(page));
  	return page;
  }
  #endif /* CONFIG_ELF_CORE */

  
  /*
   * Generic RCU Fast GUP
   *
   * get_user_pages_fast attempts to pin user pages by walking the page
   * tables directly and avoids taking locks. Thus the walker needs to be
   * protected from page table pages being freed from under it, and should
   * block any THP splits.
   *
   * One way to achieve this is to have the walker disable interrupts, and
   * rely on IPIs from the TLB flushing code blocking before the page table
   * pages are freed. This is unsuitable for architectures that do not need
   * to broadcast an IPI when invalidating TLBs.
   *
   * Another way to achieve this is to batch up page table containing pages
   * belonging to more than one mm_user, then rcu_sched a callback to free those
   * pages. Disabling interrupts will allow the fast_gup walker to both block
   * the rcu_sched callback, and an IPI that we broadcast for splitting THPs
   * (which is a relatively rare event). The code below adopts this strategy.
   *
   * Before activating this code, please be aware that the following assumptions
   * are currently made:
   *
   *  *) HAVE_RCU_TABLE_FREE is enabled, and tlb_remove_table is used to free
   *      pages containing page tables.
   *

   *  *) ptes can be read atomically by the architecture.
   *
   *  *) access_ok is sufficient to validate userspace address ranges.
   *
   * The last two assumptions can be relaxed by the addition of helper functions.
   *
   * This code is based heavily on the PowerPC implementation by Nick Piggin.
   */
  #ifdef CONFIG_HAVE_GENERIC_RCU_GUP
  
  #ifdef __HAVE_ARCH_PTE_SPECIAL
  static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
  			 int write, struct page **pages, int *nr)
  {
  	pte_t *ptep, *ptem;
  	int ret = 0;
  
  	ptem = ptep = pte_offset_map(&pmd, addr);
  	do {
  		/*
  		 * In the line below we are assuming that the pte can be read
  		 * atomically. If this is not the case for your architecture,
  		 * please wrap this in a helper function!
  		 *
  		 * for an example see gup_get_pte in arch/x86/mm/gup.c
  		 */

  		pte_t pte = READ_ONCE(*ptep);

  		struct page *head, *page;

  
  		/*
  		 * Similar to the PMD case below, NUMA hinting must take slow

  		 * path using the pte_protnone check.

  		 */
  		if (!pte_present(pte) || pte_special(pte) ||

  			pte_protnone(pte) || (write && !pte_write(pte)))

  			goto pte_unmap;

  		if (!arch_pte_access_permitted(pte, write))
  			goto pte_unmap;

  		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
  		page = pte_page(pte);

  		head = compound_head(page);

  		if (!page_cache_get_speculative(head))

  			goto pte_unmap;
  
  		if (unlikely(pte_val(pte) != pte_val(*ptep))) {

  			put_page(head);

  			goto pte_unmap;
  		}

  		VM_BUG_ON_PAGE(compound_head(page) != head, page);

  		pages[*nr] = page;
  		(*nr)++;
  
  	} while (ptep++, addr += PAGE_SIZE, addr != end);
  
  	ret = 1;
  
  pte_unmap:
  	pte_unmap(ptem);
  	return ret;
  }
  #else
  
  /*
   * If we can't determine whether or not a pte is special, then fail immediately
   * for ptes. Note, we can still pin HugeTLB and THP as these are guaranteed not
   * to be special.
   *
   * For a futex to be placed on a THP tail page, get_futex_key requires a
   * __get_user_pages_fast implementation that can pin pages. Thus it's still
   * useful to have gup_huge_pmd even if we can't operate on ptes.
   */
  static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
  			 int write, struct page **pages, int *nr)
  {
  	return 0;
  }
  #endif /* __HAVE_ARCH_PTE_SPECIAL */
  
  static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
  		unsigned long end, int write, struct page **pages, int *nr)
  {

  	struct page *head, *page;

  	int refs;
  
  	if (write && !pmd_write(orig))
  		return 0;
  
  	refs = 0;
  	head = pmd_page(orig);
  	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);

  	do {
  		VM_BUG_ON_PAGE(compound_head(page) != head, page);
  		pages[*nr] = page;
  		(*nr)++;
  		page++;
  		refs++;
  	} while (addr += PAGE_SIZE, addr != end);
  
  	if (!page_cache_add_speculative(head, refs)) {
  		*nr -= refs;
  		return 0;
  	}
  
  	if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
  		*nr -= refs;
  		while (refs--)
  			put_page(head);
  		return 0;
  	}

  	return 1;
  }
  
  static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
  		unsigned long end, int write, struct page **pages, int *nr)
  {

  	struct page *head, *page;

  	int refs;
  
  	if (write && !pud_write(orig))
  		return 0;
  
  	refs = 0;
  	head = pud_page(orig);
  	page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);

  	do {
  		VM_BUG_ON_PAGE(compound_head(page) != head, page);
  		pages[*nr] = page;
  		(*nr)++;
  		page++;
  		refs++;
  	} while (addr += PAGE_SIZE, addr != end);
  
  	if (!page_cache_add_speculative(head, refs)) {
  		*nr -= refs;
  		return 0;
  	}
  
  	if (unlikely(pud_val(orig) != pud_val(*pudp))) {
  		*nr -= refs;
  		while (refs--)
  			put_page(head);
  		return 0;
  	}

  	return 1;
  }

  static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
  			unsigned long end, int write,
  			struct page **pages, int *nr)
  {
  	int refs;

  	struct page *head, *page;

  
  	if (write && !pgd_write(orig))
  		return 0;
  
  	refs = 0;
  	head = pgd_page(orig);
  	page = head + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);

  	do {
  		VM_BUG_ON_PAGE(compound_head(page) != head, page);
  		pages[*nr] = page;
  		(*nr)++;
  		page++;
  		refs++;
  	} while (addr += PAGE_SIZE, addr != end);
  
  	if (!page_cache_add_speculative(head, refs)) {
  		*nr -= refs;
  		return 0;
  	}
  
  	if (unlikely(pgd_val(orig) != pgd_val(*pgdp))) {
  		*nr -= refs;
  		while (refs--)
  			put_page(head);
  		return 0;
  	}

  	return 1;
  }

  static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
  		int write, struct page **pages, int *nr)
  {
  	unsigned long next;
  	pmd_t *pmdp;
  
  	pmdp = pmd_offset(&pud, addr);
  	do {

  		pmd_t pmd = READ_ONCE(*pmdp);

  
  		next = pmd_addr_end(addr, end);

  		if (pmd_none(pmd))

  			return 0;
  
  		if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd))) {
  			/*
  			 * NUMA hinting faults need to be handled in the GUP
  			 * slowpath for accounting purposes and so that they
  			 * can be serialised against THP migration.
  			 */

  			if (pmd_protnone(pmd))

  				return 0;
  
  			if (!gup_huge_pmd(pmd, pmdp, addr, next, write,
  				pages, nr))
  				return 0;

  		} else if (unlikely(is_hugepd(__hugepd(pmd_val(pmd))))) {
  			/*
  			 * architecture have different format for hugetlbfs
  			 * pmd format and THP pmd format
  			 */
  			if (!gup_huge_pd(__hugepd(pmd_val(pmd)), addr,
  					 PMD_SHIFT, next, write, pages, nr))
  				return 0;

  		} else if (!gup_pte_range(pmd, addr, next, write, pages, nr))
  				return 0;
  	} while (pmdp++, addr = next, addr != end);
  
  	return 1;
  }

  static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
  			 int write, struct page **pages, int *nr)

  {
  	unsigned long next;
  	pud_t *pudp;

  	pudp = pud_offset(&pgd, addr);

  	do {

  		pud_t pud = READ_ONCE(*pudp);

  
  		next = pud_addr_end(addr, end);
  		if (pud_none(pud))
  			return 0;

  		if (unlikely(pud_huge(pud))) {

  			if (!gup_huge_pud(pud, pudp, addr, next, write,

  					  pages, nr))
  				return 0;
  		} else if (unlikely(is_hugepd(__hugepd(pud_val(pud))))) {
  			if (!gup_huge_pd(__hugepd(pud_val(pud)), addr,
  					 PUD_SHIFT, next, write, pages, nr))

  				return 0;
  		} else if (!gup_pmd_range(pud, addr, next, write, pages, nr))
  			return 0;
  	} while (pudp++, addr = next, addr != end);
  
  	return 1;
  }
  
  /*
   * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
   * the regular GUP. It will only return non-negative values.
   */
  int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
  			  struct page **pages)
  {
  	struct mm_struct *mm = current->mm;
  	unsigned long addr, len, end;
  	unsigned long next, flags;
  	pgd_t *pgdp;
  	int nr = 0;
  
  	start &= PAGE_MASK;
  	addr = start;
  	len = (unsigned long) nr_pages << PAGE_SHIFT;
  	end = start + len;
  
  	if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
  					start, len)))
  		return 0;
  
  	/*
  	 * Disable interrupts.  We use the nested form as we can already have
  	 * interrupts disabled by get_futex_key.
  	 *
  	 * With interrupts disabled, we block page table pages from being
  	 * freed from under us. See mmu_gather_tlb in asm-generic/tlb.h
  	 * for more details.
  	 *
  	 * We do not adopt an rcu_read_lock(.) here as we also want to
  	 * block IPIs that come from THPs splitting.
  	 */
  
  	local_irq_save(flags);
  	pgdp = pgd_offset(mm, addr);
  	do {

  		pgd_t pgd = READ_ONCE(*pgdp);

  		next = pgd_addr_end(addr, end);

  		if (pgd_none(pgd))

  			break;

  		if (unlikely(pgd_huge(pgd))) {
  			if (!gup_huge_pgd(pgd, pgdp, addr, next, write,
  					  pages, &nr))
  				break;
  		} else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
  			if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
  					 PGDIR_SHIFT, next, write, pages, &nr))
  				break;
  		} else if (!gup_pud_range(pgd, addr, next, write, pages, &nr))

  			break;
  	} while (pgdp++, addr = next, addr != end);
  	local_irq_restore(flags);
  
  	return nr;
  }
  
  /**
   * get_user_pages_fast() - pin user pages in memory
   * @start:	starting user address
   * @nr_pages:	number of pages from start to pin
   * @write:	whether pages will be written to
   * @pages:	array that receives pointers to the pages pinned.
   *		Should be at least nr_pages long.
   *
   * Attempt to pin user pages in memory without taking mm->mmap_sem.
   * If not successful, it will fall back to taking the lock and
   * calling get_user_pages().
   *
   * Returns number of pages pinned. This may be fewer than the number
   * requested. If nr_pages is 0 or negative, returns 0. If no pages
   * were pinned, returns -errno.
   */
  int get_user_pages_fast(unsigned long start, int nr_pages, int write,
  			struct page **pages)
  {

  	int nr, ret;
  
  	start &= PAGE_MASK;
  	nr = __get_user_pages_fast(start, nr_pages, write, pages);
  	ret = nr;
  
  	if (nr < nr_pages) {
  		/* Try to get the remaining pages with get_user_pages */
  		start += nr << PAGE_SHIFT;
  		pages += nr;

  		ret = get_user_pages_unlocked(start, nr_pages - nr, pages,
  				write ? FOLL_WRITE : 0);

  
  		/* Have to be a bit careful with return values */
  		if (nr > 0) {
  			if (ret < 0)
  				ret = nr;
  			else
  				ret += nr;
  		}
  	}
  
  	return ret;
  }
  
  #endif /* CONFIG_HAVE_GENERIC_RCU_GUP */