text: #include <linux/kernel.h>
  #include <linux/errno.h>
  #include <linux/err.h>
  #include <linux/mm.h>
  #include <linux/slab.h>
  #include <linux/vmalloc.h>
  #include <linux/pagemap.h>
  #include <linux/sched.h>
  
  /*
   * get_vaddr_frames() - map virtual addresses to pfns
   * @start:	starting user address
   * @nr_frames:	number of pages / pfns from start to map
   * @write:	whether pages will be written to by the caller
   * @force:	whether to force write access even if user mapping is
   *		readonly. See description of the same argument of
  		get_user_pages().
   * @vec:	structure which receives pages / pfns of the addresses mapped.
   *		It should have space for at least nr_frames entries.
   *
   * This function maps virtual addresses from @start and fills @vec structure
   * with page frame numbers or page pointers to corresponding pages (choice
   * depends on the type of the vma underlying the virtual address). If @start
   * belongs to a normal vma, the function grabs reference to each of the pages
   * to pin them in memory. If @start belongs to VM_IO | VM_PFNMAP vma, we don't
   * touch page structures and the caller must make sure pfns aren't reused for
   * anything else while he is using them.
   *
   * The function returns number of pages mapped which may be less than
   * @nr_frames. In particular we stop mapping if there are more vmas of
   * different type underlying the specified range of virtual addresses.
   * When the function isn't able to map a single page, it returns error.
   *
   * This function takes care of grabbing mmap_sem as necessary.
   */
  int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
  		     bool write, bool force, struct frame_vector *vec)
  {
  	struct mm_struct *mm = current->mm;
  	struct vm_area_struct *vma;
  	int ret = 0;
  	int err;
  	int locked;
  
  	if (nr_frames == 0)
  		return 0;
  
  	if (WARN_ON_ONCE(nr_frames > vec->nr_allocated))
  		nr_frames = vec->nr_allocated;
  
  	down_read(&mm->mmap_sem);
  	locked = 1;
  	vma = find_vma_intersection(mm, start, start + 1);
  	if (!vma) {
  		ret = -EFAULT;
  		goto out;
  	}
  	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
  		vec->got_ref = true;
  		vec->is_pfns = false;
  		ret = get_user_pages_locked(current, mm, start, nr_frames,
  			write, force, (struct page **)(vec->ptrs), &locked);
  		goto out;
  	}
  
  	vec->got_ref = false;
  	vec->is_pfns = true;
  	do {
  		unsigned long *nums = frame_vector_pfns(vec);
  
  		while (ret < nr_frames && start + PAGE_SIZE <= vma->vm_end) {
  			err = follow_pfn(vma, start, &nums[ret]);
  			if (err) {
  				if (ret == 0)
  					ret = err;
  				goto out;
  			}
  			start += PAGE_SIZE;
  			ret++;
  		}
  		/*
  		 * We stop if we have enough pages or if VMA doesn't completely
  		 * cover the tail page.
  		 */
  		if (ret >= nr_frames || start < vma->vm_end)
  			break;
  		vma = find_vma_intersection(mm, start, start + 1);
  	} while (vma && vma->vm_flags & (VM_IO | VM_PFNMAP));
  out:
  	if (locked)
  		up_read(&mm->mmap_sem);
  	if (!ret)
  		ret = -EFAULT;
  	if (ret > 0)
  		vec->nr_frames = ret;
  	return ret;
  }
  EXPORT_SYMBOL(get_vaddr_frames);
  
  /**
   * put_vaddr_frames() - drop references to pages if get_vaddr_frames() acquired
   *			them
   * @vec:	frame vector to put
   *
   * Drop references to pages if get_vaddr_frames() acquired them. We also
   * invalidate the frame vector so that it is prepared for the next call into
   * get_vaddr_frames().
   */
  void put_vaddr_frames(struct frame_vector *vec)
  {
  	int i;
  	struct page **pages;
  
  	if (!vec->got_ref)
  		goto out;
  	pages = frame_vector_pages(vec);
  	/*
  	 * frame_vector_pages() might needed to do a conversion when
  	 * get_vaddr_frames() got pages but vec was later converted to pfns.
  	 * But it shouldn't really fail to convert pfns back...
  	 */
  	if (WARN_ON(IS_ERR(pages)))
  		goto out;
  	for (i = 0; i < vec->nr_frames; i++)
  		put_page(pages[i]);
  	vec->got_ref = false;
  out:
  	vec->nr_frames = 0;
  }
  EXPORT_SYMBOL(put_vaddr_frames);
  
  /**
   * frame_vector_to_pages - convert frame vector to contain page pointers
   * @vec:	frame vector to convert
   *
   * Convert @vec to contain array of page pointers.  If the conversion is
   * successful, return 0. Otherwise return an error. Note that we do not grab
   * page references for the page structures.
   */
  int frame_vector_to_pages(struct frame_vector *vec)
  {
  	int i;
  	unsigned long *nums;
  	struct page **pages;
  
  	if (!vec->is_pfns)
  		return 0;
  	nums = frame_vector_pfns(vec);
  	for (i = 0; i < vec->nr_frames; i++)
  		if (!pfn_valid(nums[i]))
  			return -EINVAL;
  	pages = (struct page **)nums;
  	for (i = 0; i < vec->nr_frames; i++)
  		pages[i] = pfn_to_page(nums[i]);
  	vec->is_pfns = false;
  	return 0;
  }
  EXPORT_SYMBOL(frame_vector_to_pages);
  
  /**
   * frame_vector_to_pfns - convert frame vector to contain pfns
   * @vec:	frame vector to convert
   *
   * Convert @vec to contain array of pfns.
   */
  void frame_vector_to_pfns(struct frame_vector *vec)
  {
  	int i;
  	unsigned long *nums;
  	struct page **pages;
  
  	if (vec->is_pfns)
  		return;
  	pages = (struct page **)(vec->ptrs);
  	nums = (unsigned long *)pages;
  	for (i = 0; i < vec->nr_frames; i++)
  		nums[i] = page_to_pfn(pages[i]);
  	vec->is_pfns = true;
  }
  EXPORT_SYMBOL(frame_vector_to_pfns);
  
  /**
   * frame_vector_create() - allocate & initialize structure for pinned pfns
   * @nr_frames:	number of pfns slots we should reserve
   *
   * Allocate and initialize struct pinned_pfns to be able to hold @nr_pfns
   * pfns.
   */
  struct frame_vector *frame_vector_create(unsigned int nr_frames)
  {
  	struct frame_vector *vec;
  	int size = sizeof(struct frame_vector) + sizeof(void *) * nr_frames;
  
  	if (WARN_ON_ONCE(nr_frames == 0))
  		return NULL;
  	/*
  	 * This is absurdly high. It's here just to avoid strange effects when
  	 * arithmetics overflows.
  	 */
  	if (WARN_ON_ONCE(nr_frames > INT_MAX / sizeof(void *) / 2))
  		return NULL;
  	/*
  	 * Avoid higher order allocations, use vmalloc instead. It should
  	 * be rare anyway.
  	 */
  	if (size <= PAGE_SIZE)
  		vec = kmalloc(size, GFP_KERNEL);
  	else
  		vec = vmalloc(size);
  	if (!vec)
  		return NULL;
  	vec->nr_allocated = nr_frames;
  	vec->nr_frames = 0;
  	return vec;
  }
  EXPORT_SYMBOL(frame_vector_create);
  
  /**
   * frame_vector_destroy() - free memory allocated to carry frame vector
   * @vec:	Frame vector to free
   *
   * Free structure allocated by frame_vector_create() to carry frames.
   */
  void frame_vector_destroy(struct frame_vector *vec)
  {
  	/* Make sure put_vaddr_frames() got called properly... */
  	VM_BUG_ON(vec->nr_frames > 0);
  	kvfree(vec);
  }
  EXPORT_SYMBOL(frame_vector_destroy);
  