#include <linux/export.h>
  #include <linux/uio.h>
  #include <linux/pagemap.h>

  #include <linux/slab.h>
  #include <linux/vmalloc.h>

  #include <net/checksum.h>

  #define iterate_iovec(i, n, __v, __p, skip, STEP) {	\
  	size_t left;					\
  	size_t wanted = n;				\
  	__p = i->iov;					\
  	__v.iov_len = min(n, __p->iov_len - skip);	\
  	if (likely(__v.iov_len)) {			\
  		__v.iov_base = __p->iov_base + skip;	\
  		left = (STEP);				\
  		__v.iov_len -= left;			\
  		skip += __v.iov_len;			\
  		n -= __v.iov_len;			\
  	} else {					\
  		left = 0;				\
  	}						\
  	while (unlikely(!left && n)) {			\
  		__p++;					\
  		__v.iov_len = min(n, __p->iov_len);	\
  		if (unlikely(!__v.iov_len))		\
  			continue;			\
  		__v.iov_base = __p->iov_base;		\
  		left = (STEP);				\
  		__v.iov_len -= left;			\
  		skip = __v.iov_len;			\
  		n -= __v.iov_len;			\
  	}						\
  	n = wanted - n;					\
  }

  #define iterate_kvec(i, n, __v, __p, skip, STEP) {	\
  	size_t wanted = n;				\
  	__p = i->kvec;					\
  	__v.iov_len = min(n, __p->iov_len - skip);	\
  	if (likely(__v.iov_len)) {			\
  		__v.iov_base = __p->iov_base + skip;	\
  		(void)(STEP);				\
  		skip += __v.iov_len;			\
  		n -= __v.iov_len;			\
  	}						\
  	while (unlikely(n)) {				\
  		__p++;					\
  		__v.iov_len = min(n, __p->iov_len);	\
  		if (unlikely(!__v.iov_len))		\
  			continue;			\
  		__v.iov_base = __p->iov_base;		\
  		(void)(STEP);				\
  		skip = __v.iov_len;			\
  		n -= __v.iov_len;			\
  	}						\
  	n = wanted;					\
  }

  #define iterate_bvec(i, n, __v, __bi, skip, STEP) {	\
  	struct bvec_iter __start;			\
  	__start.bi_size = n;				\
  	__start.bi_bvec_done = skip;			\
  	__start.bi_idx = 0;				\
  	for_each_bvec(__v, i->bvec, __bi, __start) {	\
  		if (!__v.bv_len)			\

  			continue;			\

  		(void)(STEP);				\

  	}						\

  }

  #define iterate_all_kinds(i, n, v, I, B, K) {			\

  	size_t skip = i->iov_offset;				\
  	if (unlikely(i->type & ITER_BVEC)) {			\

  		struct bio_vec v;				\

  		struct bvec_iter __bi;				\
  		iterate_bvec(i, n, v, __bi, skip, (B))		\

  	} else if (unlikely(i->type & ITER_KVEC)) {		\
  		const struct kvec *kvec;			\
  		struct kvec v;					\
  		iterate_kvec(i, n, v, kvec, skip, (K))		\

  	} else {						\
  		const struct iovec *iov;			\
  		struct iovec v;					\
  		iterate_iovec(i, n, v, iov, skip, (I))		\
  	}							\
  }

  #define iterate_and_advance(i, n, v, I, B, K) {			\

  	if (unlikely(i->count < n))				\
  		n = i->count;					\

  	if (i->count) {						\

  		size_t skip = i->iov_offset;			\
  		if (unlikely(i->type & ITER_BVEC)) {		\

  			const struct bio_vec *bvec = i->bvec;	\

  			struct bio_vec v;			\

  			struct bvec_iter __bi;			\
  			iterate_bvec(i, n, v, __bi, skip, (B))	\
  			i->bvec = __bvec_iter_bvec(i->bvec, __bi);	\
  			i->nr_segs -= i->bvec - bvec;		\
  			skip = __bi.bi_bvec_done;		\

  		} else if (unlikely(i->type & ITER_KVEC)) {	\
  			const struct kvec *kvec;		\
  			struct kvec v;				\
  			iterate_kvec(i, n, v, kvec, skip, (K))	\
  			if (skip == kvec->iov_len) {		\
  				kvec++;				\
  				skip = 0;			\
  			}					\
  			i->nr_segs -= kvec - i->kvec;		\
  			i->kvec = kvec;				\
  		} else {					\
  			const struct iovec *iov;		\
  			struct iovec v;				\
  			iterate_iovec(i, n, v, iov, skip, (I))	\
  			if (skip == iov->iov_len) {		\
  				iov++;				\
  				skip = 0;			\
  			}					\
  			i->nr_segs -= iov - i->iov;		\
  			i->iov = iov;				\

  		}						\

  		i->count -= n;					\
  		i->iov_offset = skip;				\

  	}							\

  }

  static size_t copy_page_to_iter_iovec(struct page *page, size_t offset, size_t bytes,

  			 struct iov_iter *i)
  {
  	size_t skip, copy, left, wanted;
  	const struct iovec *iov;
  	char __user *buf;
  	void *kaddr, *from;
  
  	if (unlikely(bytes > i->count))
  		bytes = i->count;
  
  	if (unlikely(!bytes))
  		return 0;
  
  	wanted = bytes;
  	iov = i->iov;
  	skip = i->iov_offset;
  	buf = iov->iov_base + skip;
  	copy = min(bytes, iov->iov_len - skip);

  	if (IS_ENABLED(CONFIG_HIGHMEM) && !fault_in_pages_writeable(buf, copy)) {

  		kaddr = kmap_atomic(page);
  		from = kaddr + offset;
  
  		/* first chunk, usually the only one */
  		left = __copy_to_user_inatomic(buf, from, copy);
  		copy -= left;
  		skip += copy;
  		from += copy;
  		bytes -= copy;
  
  		while (unlikely(!left && bytes)) {
  			iov++;
  			buf = iov->iov_base;
  			copy = min(bytes, iov->iov_len);
  			left = __copy_to_user_inatomic(buf, from, copy);
  			copy -= left;
  			skip = copy;
  			from += copy;
  			bytes -= copy;
  		}
  		if (likely(!bytes)) {
  			kunmap_atomic(kaddr);
  			goto done;
  		}
  		offset = from - kaddr;
  		buf += copy;
  		kunmap_atomic(kaddr);
  		copy = min(bytes, iov->iov_len - skip);
  	}
  	/* Too bad - revert to non-atomic kmap */

  	kaddr = kmap(page);
  	from = kaddr + offset;
  	left = __copy_to_user(buf, from, copy);
  	copy -= left;
  	skip += copy;
  	from += copy;
  	bytes -= copy;
  	while (unlikely(!left && bytes)) {
  		iov++;
  		buf = iov->iov_base;
  		copy = min(bytes, iov->iov_len);
  		left = __copy_to_user(buf, from, copy);
  		copy -= left;
  		skip = copy;
  		from += copy;
  		bytes -= copy;
  	}
  	kunmap(page);

  done:

  	if (skip == iov->iov_len) {
  		iov++;
  		skip = 0;
  	}

  	i->count -= wanted - bytes;
  	i->nr_segs -= iov - i->iov;
  	i->iov = iov;
  	i->iov_offset = skip;
  	return wanted - bytes;
  }

  static size_t copy_page_from_iter_iovec(struct page *page, size_t offset, size_t bytes,

  			 struct iov_iter *i)
  {
  	size_t skip, copy, left, wanted;
  	const struct iovec *iov;
  	char __user *buf;
  	void *kaddr, *to;
  
  	if (unlikely(bytes > i->count))
  		bytes = i->count;
  
  	if (unlikely(!bytes))
  		return 0;
  
  	wanted = bytes;
  	iov = i->iov;
  	skip = i->iov_offset;
  	buf = iov->iov_base + skip;
  	copy = min(bytes, iov->iov_len - skip);

  	if (IS_ENABLED(CONFIG_HIGHMEM) && !fault_in_pages_readable(buf, copy)) {

  		kaddr = kmap_atomic(page);
  		to = kaddr + offset;
  
  		/* first chunk, usually the only one */
  		left = __copy_from_user_inatomic(to, buf, copy);
  		copy -= left;
  		skip += copy;
  		to += copy;
  		bytes -= copy;
  
  		while (unlikely(!left && bytes)) {
  			iov++;
  			buf = iov->iov_base;
  			copy = min(bytes, iov->iov_len);
  			left = __copy_from_user_inatomic(to, buf, copy);
  			copy -= left;
  			skip = copy;
  			to += copy;
  			bytes -= copy;
  		}
  		if (likely(!bytes)) {
  			kunmap_atomic(kaddr);
  			goto done;
  		}
  		offset = to - kaddr;
  		buf += copy;
  		kunmap_atomic(kaddr);
  		copy = min(bytes, iov->iov_len - skip);
  	}
  	/* Too bad - revert to non-atomic kmap */

  	kaddr = kmap(page);
  	to = kaddr + offset;
  	left = __copy_from_user(to, buf, copy);
  	copy -= left;
  	skip += copy;
  	to += copy;
  	bytes -= copy;
  	while (unlikely(!left && bytes)) {
  		iov++;
  		buf = iov->iov_base;
  		copy = min(bytes, iov->iov_len);
  		left = __copy_from_user(to, buf, copy);
  		copy -= left;
  		skip = copy;
  		to += copy;
  		bytes -= copy;
  	}
  	kunmap(page);

  done:

  	if (skip == iov->iov_len) {
  		iov++;
  		skip = 0;
  	}

  	i->count -= wanted - bytes;
  	i->nr_segs -= iov - i->iov;
  	i->iov = iov;
  	i->iov_offset = skip;
  	return wanted - bytes;
  }

  /*

   * Fault in one or more iovecs of the given iov_iter, to a maximum length of
   * bytes.  For each iovec, fault in each page that constitutes the iovec.
   *
   * Return 0 on success, or non-zero if the memory could not be accessed (i.e.
   * because it is an invalid address).
   */

  int iov_iter_fault_in_readable(struct iov_iter *i, size_t bytes)

  {
  	size_t skip = i->iov_offset;
  	const struct iovec *iov;
  	int err;
  	struct iovec v;
  
  	if (!(i->type & (ITER_BVEC|ITER_KVEC))) {
  		iterate_iovec(i, bytes, v, iov, skip, ({
  			err = fault_in_multipages_readable(v.iov_base,
  					v.iov_len);
  			if (unlikely(err))
  			return err;
  		0;}))
  	}
  	return 0;
  }

  EXPORT_SYMBOL(iov_iter_fault_in_readable);

  void iov_iter_init(struct iov_iter *i, int direction,
  			const struct iovec *iov, unsigned long nr_segs,
  			size_t count)
  {
  	/* It will get better.  Eventually... */

  	if (segment_eq(get_fs(), KERNEL_DS)) {

  		direction |= ITER_KVEC;

  		i->type = direction;
  		i->kvec = (struct kvec *)iov;
  	} else {
  		i->type = direction;
  		i->iov = iov;
  	}

  	i->nr_segs = nr_segs;
  	i->iov_offset = 0;
  	i->count = count;
  }
  EXPORT_SYMBOL(iov_iter_init);

  static void memcpy_from_page(char *to, struct page *page, size_t offset, size_t len)
  {
  	char *from = kmap_atomic(page);
  	memcpy(to, from + offset, len);
  	kunmap_atomic(from);
  }

  static void memcpy_to_page(struct page *page, size_t offset, const char *from, size_t len)

  {
  	char *to = kmap_atomic(page);
  	memcpy(to + offset, from, len);
  	kunmap_atomic(to);
  }

  static void memzero_page(struct page *page, size_t offset, size_t len)
  {
  	char *addr = kmap_atomic(page);
  	memset(addr + offset, 0, len);
  	kunmap_atomic(addr);
  }

  size_t copy_to_iter(const void *addr, size_t bytes, struct iov_iter *i)

  {

  	const char *from = addr;

  	iterate_and_advance(i, bytes, v,
  		__copy_to_user(v.iov_base, (from += v.iov_len) - v.iov_len,
  			       v.iov_len),
  		memcpy_to_page(v.bv_page, v.bv_offset,

  			       (from += v.bv_len) - v.bv_len, v.bv_len),
  		memcpy(v.iov_base, (from += v.iov_len) - v.iov_len, v.iov_len)

  	)

  	return bytes;

  }

  EXPORT_SYMBOL(copy_to_iter);

  size_t copy_from_iter(void *addr, size_t bytes, struct iov_iter *i)

  {

  	char *to = addr;

  	iterate_and_advance(i, bytes, v,
  		__copy_from_user((to += v.iov_len) - v.iov_len, v.iov_base,
  				 v.iov_len),
  		memcpy_from_page((to += v.bv_len) - v.bv_len, v.bv_page,

  				 v.bv_offset, v.bv_len),
  		memcpy((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len)

  	)
  
  	return bytes;

  }

  EXPORT_SYMBOL(copy_from_iter);

  size_t copy_from_iter_nocache(void *addr, size_t bytes, struct iov_iter *i)
  {
  	char *to = addr;

  	iterate_and_advance(i, bytes, v,
  		__copy_from_user_nocache((to += v.iov_len) - v.iov_len,
  					 v.iov_base, v.iov_len),
  		memcpy_from_page((to += v.bv_len) - v.bv_len, v.bv_page,
  				 v.bv_offset, v.bv_len),
  		memcpy((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len)
  	)
  
  	return bytes;
  }
  EXPORT_SYMBOL(copy_from_iter_nocache);

  size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
  			 struct iov_iter *i)
  {

  	if (i->type & (ITER_BVEC|ITER_KVEC)) {
  		void *kaddr = kmap_atomic(page);
  		size_t wanted = copy_to_iter(kaddr + offset, bytes, i);
  		kunmap_atomic(kaddr);
  		return wanted;
  	} else

  		return copy_page_to_iter_iovec(page, offset, bytes, i);
  }
  EXPORT_SYMBOL(copy_page_to_iter);
  
  size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
  			 struct iov_iter *i)
  {

  	if (i->type & (ITER_BVEC|ITER_KVEC)) {

  		void *kaddr = kmap_atomic(page);
  		size_t wanted = copy_from_iter(kaddr + offset, bytes, i);
  		kunmap_atomic(kaddr);
  		return wanted;
  	} else

  		return copy_page_from_iter_iovec(page, offset, bytes, i);
  }
  EXPORT_SYMBOL(copy_page_from_iter);

  size_t iov_iter_zero(size_t bytes, struct iov_iter *i)
  {

  	iterate_and_advance(i, bytes, v,
  		__clear_user(v.iov_base, v.iov_len),

  		memzero_page(v.bv_page, v.bv_offset, v.bv_len),
  		memset(v.iov_base, 0, v.iov_len)

  	)
  
  	return bytes;

  }
  EXPORT_SYMBOL(iov_iter_zero);

  size_t iov_iter_copy_from_user_atomic(struct page *page,
  		struct iov_iter *i, unsigned long offset, size_t bytes)
  {

  	char *kaddr = kmap_atomic(page), *p = kaddr + offset;
  	iterate_all_kinds(i, bytes, v,
  		__copy_from_user_inatomic((p += v.iov_len) - v.iov_len,
  					  v.iov_base, v.iov_len),
  		memcpy_from_page((p += v.bv_len) - v.bv_len, v.bv_page,

  				 v.bv_offset, v.bv_len),
  		memcpy((p += v.iov_len) - v.iov_len, v.iov_base, v.iov_len)

  	)
  	kunmap_atomic(kaddr);
  	return bytes;

  }
  EXPORT_SYMBOL(iov_iter_copy_from_user_atomic);
  
  void iov_iter_advance(struct iov_iter *i, size_t size)
  {

  	iterate_and_advance(i, size, v, 0, 0, 0)

  }
  EXPORT_SYMBOL(iov_iter_advance);
  
  /*
   * Return the count of just the current iov_iter segment.
   */
  size_t iov_iter_single_seg_count(const struct iov_iter *i)
  {
  	if (i->nr_segs == 1)
  		return i->count;
  	else if (i->type & ITER_BVEC)

  		return min(i->count, i->bvec->bv_len - i->iov_offset);

  	else
  		return min(i->count, i->iov->iov_len - i->iov_offset);

  }
  EXPORT_SYMBOL(iov_iter_single_seg_count);

  void iov_iter_kvec(struct iov_iter *i, int direction,

  			const struct kvec *kvec, unsigned long nr_segs,

  			size_t count)
  {
  	BUG_ON(!(direction & ITER_KVEC));
  	i->type = direction;

  	i->kvec = kvec;

  	i->nr_segs = nr_segs;
  	i->iov_offset = 0;
  	i->count = count;
  }
  EXPORT_SYMBOL(iov_iter_kvec);

  void iov_iter_bvec(struct iov_iter *i, int direction,
  			const struct bio_vec *bvec, unsigned long nr_segs,
  			size_t count)
  {
  	BUG_ON(!(direction & ITER_BVEC));
  	i->type = direction;
  	i->bvec = bvec;
  	i->nr_segs = nr_segs;
  	i->iov_offset = 0;
  	i->count = count;
  }
  EXPORT_SYMBOL(iov_iter_bvec);

  unsigned long iov_iter_alignment(const struct iov_iter *i)
  {

  	unsigned long res = 0;
  	size_t size = i->count;
  
  	if (!size)
  		return 0;
  
  	iterate_all_kinds(i, size, v,
  		(res |= (unsigned long)v.iov_base | v.iov_len, 0),

  		res |= v.bv_offset | v.bv_len,
  		res |= (unsigned long)v.iov_base | v.iov_len

  	)
  	return res;

  }
  EXPORT_SYMBOL(iov_iter_alignment);

  unsigned long iov_iter_gap_alignment(const struct iov_iter *i)
  {
          unsigned long res = 0;
  	size_t size = i->count;
  	if (!size)
  		return 0;
  
  	iterate_all_kinds(i, size, v,
  		(res |= (!res ? 0 : (unsigned long)v.iov_base) |
  			(size != v.iov_len ? size : 0), 0),
  		(res |= (!res ? 0 : (unsigned long)v.bv_offset) |
  			(size != v.bv_len ? size : 0)),
  		(res |= (!res ? 0 : (unsigned long)v.iov_base) |
  			(size != v.iov_len ? size : 0))
  		);
  		return res;
  }
  EXPORT_SYMBOL(iov_iter_gap_alignment);

  ssize_t iov_iter_get_pages(struct iov_iter *i,

  		   struct page **pages, size_t maxsize, unsigned maxpages,

  		   size_t *start)
  {

  	if (maxsize > i->count)
  		maxsize = i->count;
  
  	if (!maxsize)
  		return 0;
  
  	iterate_all_kinds(i, maxsize, v, ({
  		unsigned long addr = (unsigned long)v.iov_base;
  		size_t len = v.iov_len + (*start = addr & (PAGE_SIZE - 1));
  		int n;
  		int res;
  
  		if (len > maxpages * PAGE_SIZE)
  			len = maxpages * PAGE_SIZE;
  		addr &= ~(PAGE_SIZE - 1);
  		n = DIV_ROUND_UP(len, PAGE_SIZE);
  		res = get_user_pages_fast(addr, n, (i->type & WRITE) != WRITE, pages);
  		if (unlikely(res < 0))
  			return res;
  		return (res == n ? len : res * PAGE_SIZE) - *start;
  	0;}),({
  		/* can't be more than PAGE_SIZE */
  		*start = v.bv_offset;
  		get_page(*pages = v.bv_page);
  		return v.bv_len;

  	}),({
  		return -EFAULT;

  	})
  	)
  	return 0;

  }
  EXPORT_SYMBOL(iov_iter_get_pages);

  static struct page **get_pages_array(size_t n)
  {
  	struct page **p = kmalloc(n * sizeof(struct page *), GFP_KERNEL);
  	if (!p)
  		p = vmalloc(n * sizeof(struct page *));
  	return p;
  }

  ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
  		   struct page ***pages, size_t maxsize,
  		   size_t *start)
  {

  	struct page **p;
  
  	if (maxsize > i->count)
  		maxsize = i->count;
  
  	if (!maxsize)
  		return 0;
  
  	iterate_all_kinds(i, maxsize, v, ({
  		unsigned long addr = (unsigned long)v.iov_base;
  		size_t len = v.iov_len + (*start = addr & (PAGE_SIZE - 1));
  		int n;
  		int res;
  
  		addr &= ~(PAGE_SIZE - 1);
  		n = DIV_ROUND_UP(len, PAGE_SIZE);
  		p = get_pages_array(n);
  		if (!p)
  			return -ENOMEM;
  		res = get_user_pages_fast(addr, n, (i->type & WRITE) != WRITE, p);
  		if (unlikely(res < 0)) {
  			kvfree(p);
  			return res;
  		}
  		*pages = p;
  		return (res == n ? len : res * PAGE_SIZE) - *start;
  	0;}),({
  		/* can't be more than PAGE_SIZE */
  		*start = v.bv_offset;
  		*pages = p = get_pages_array(1);
  		if (!p)
  			return -ENOMEM;
  		get_page(*p = v.bv_page);
  		return v.bv_len;

  	}),({
  		return -EFAULT;

  	})
  	)
  	return 0;

  }
  EXPORT_SYMBOL(iov_iter_get_pages_alloc);

  size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum,
  			       struct iov_iter *i)
  {
  	char *to = addr;
  	__wsum sum, next;
  	size_t off = 0;

  	sum = *csum;
  	iterate_and_advance(i, bytes, v, ({
  		int err = 0;
  		next = csum_and_copy_from_user(v.iov_base, 
  					       (to += v.iov_len) - v.iov_len,
  					       v.iov_len, 0, &err);
  		if (!err) {
  			sum = csum_block_add(sum, next, off);
  			off += v.iov_len;
  		}
  		err ? v.iov_len : 0;
  	}), ({
  		char *p = kmap_atomic(v.bv_page);
  		next = csum_partial_copy_nocheck(p + v.bv_offset,
  						 (to += v.bv_len) - v.bv_len,
  						 v.bv_len, 0);
  		kunmap_atomic(p);
  		sum = csum_block_add(sum, next, off);
  		off += v.bv_len;
  	}),({
  		next = csum_partial_copy_nocheck(v.iov_base,
  						 (to += v.iov_len) - v.iov_len,
  						 v.iov_len, 0);
  		sum = csum_block_add(sum, next, off);
  		off += v.iov_len;
  	})
  	)
  	*csum = sum;
  	return bytes;
  }
  EXPORT_SYMBOL(csum_and_copy_from_iter);

  size_t csum_and_copy_to_iter(const void *addr, size_t bytes, __wsum *csum,

  			     struct iov_iter *i)
  {

  	const char *from = addr;

  	__wsum sum, next;
  	size_t off = 0;

  	sum = *csum;
  	iterate_and_advance(i, bytes, v, ({
  		int err = 0;
  		next = csum_and_copy_to_user((from += v.iov_len) - v.iov_len,
  					     v.iov_base, 
  					     v.iov_len, 0, &err);
  		if (!err) {
  			sum = csum_block_add(sum, next, off);
  			off += v.iov_len;
  		}
  		err ? v.iov_len : 0;
  	}), ({
  		char *p = kmap_atomic(v.bv_page);
  		next = csum_partial_copy_nocheck((from += v.bv_len) - v.bv_len,
  						 p + v.bv_offset,
  						 v.bv_len, 0);
  		kunmap_atomic(p);
  		sum = csum_block_add(sum, next, off);
  		off += v.bv_len;
  	}),({
  		next = csum_partial_copy_nocheck((from += v.iov_len) - v.iov_len,
  						 v.iov_base,
  						 v.iov_len, 0);
  		sum = csum_block_add(sum, next, off);
  		off += v.iov_len;
  	})
  	)
  	*csum = sum;
  	return bytes;
  }
  EXPORT_SYMBOL(csum_and_copy_to_iter);

  int iov_iter_npages(const struct iov_iter *i, int maxpages)
  {

  	size_t size = i->count;
  	int npages = 0;
  
  	if (!size)
  		return 0;
  
  	iterate_all_kinds(i, size, v, ({
  		unsigned long p = (unsigned long)v.iov_base;
  		npages += DIV_ROUND_UP(p + v.iov_len, PAGE_SIZE)
  			- p / PAGE_SIZE;
  		if (npages >= maxpages)
  			return maxpages;
  	0;}),({
  		npages++;
  		if (npages >= maxpages)
  			return maxpages;

  	}),({
  		unsigned long p = (unsigned long)v.iov_base;
  		npages += DIV_ROUND_UP(p + v.iov_len, PAGE_SIZE)
  			- p / PAGE_SIZE;
  		if (npages >= maxpages)
  			return maxpages;

  	})
  	)
  	return npages;

  }

  EXPORT_SYMBOL(iov_iter_npages);

  
  const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags)
  {
  	*new = *old;
  	if (new->type & ITER_BVEC)
  		return new->bvec = kmemdup(new->bvec,
  				    new->nr_segs * sizeof(struct bio_vec),
  				    flags);
  	else
  		/* iovec and kvec have identical layout */
  		return new->iov = kmemdup(new->iov,
  				   new->nr_segs * sizeof(struct iovec),
  				   flags);
  }
  EXPORT_SYMBOL(dup_iter);

  
  int import_iovec(int type, const struct iovec __user * uvector,
  		 unsigned nr_segs, unsigned fast_segs,
  		 struct iovec **iov, struct iov_iter *i)
  {
  	ssize_t n;
  	struct iovec *p;
  	n = rw_copy_check_uvector(type, uvector, nr_segs, fast_segs,
  				  *iov, &p);
  	if (n < 0) {
  		if (p != *iov)
  			kfree(p);
  		*iov = NULL;
  		return n;
  	}
  	iov_iter_init(i, type, p, nr_segs, n);
  	*iov = p == *iov ? NULL : p;
  	return 0;
  }
  EXPORT_SYMBOL(import_iovec);
  
  #ifdef CONFIG_COMPAT
  #include <linux/compat.h>
  
  int compat_import_iovec(int type, const struct compat_iovec __user * uvector,
  		 unsigned nr_segs, unsigned fast_segs,
  		 struct iovec **iov, struct iov_iter *i)
  {
  	ssize_t n;
  	struct iovec *p;
  	n = compat_rw_copy_check_uvector(type, uvector, nr_segs, fast_segs,
  				  *iov, &p);
  	if (n < 0) {
  		if (p != *iov)
  			kfree(p);
  		*iov = NULL;
  		return n;
  	}
  	iov_iter_init(i, type, p, nr_segs, n);
  	*iov = p == *iov ? NULL : p;
  	return 0;
  }
  #endif
  
  int import_single_range(int rw, void __user *buf, size_t len,
  		 struct iovec *iov, struct iov_iter *i)
  {
  	if (len > MAX_RW_COUNT)
  		len = MAX_RW_COUNT;
  	if (unlikely(!access_ok(!rw, buf, len)))
  		return -EFAULT;
  
  	iov->iov_base = buf;
  	iov->iov_len = len;
  	iov_iter_init(i, rw, iov, 1, len);
  	return 0;
  }

  EXPORT_SYMBOL(import_single_range);