cifs_iovec_read: keep iov_iter between the calls of cifs_readdata_to_iov()

... we are doing them on adjacent parts of file, so what happens is that
each subsequent call works to rebuild the iov_iter to exact state it
had been abandoned in by previous one.  Just keep it through the entire
cifs_iovec_read().  And use copy_page_to_iter() instead of doing
kmap/copy_to_user/kunmap manually...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/cifs/file.c

@@ -2727,56 +2727,27 @@ cifs_retry_async_readv(struct cifs_readdata *rdata)
 /**
  * cifs_readdata_to_iov - copy data from pages in response to an iovec
  * @rdata:	the readdata response with list of pages holding data
- * @iov:	vector in which we should copy the data
- * @nr_segs:	number of segments in vector
- * @offset:	offset into file of the first iovec
- * @copied:	used to return the amount of data copied to the iov
+ * @iter:	destination for our data
  *
  * This function copies data from a list of pages in a readdata response into
  * an array of iovecs. It will first calculate where the data should go
  * based on the info in the readdata and then copy the data into that spot.
  */
-static ssize_t
-cifs_readdata_to_iov(struct cifs_readdata *rdata, const struct iovec *iov,
-			unsigned long nr_segs, loff_t offset, ssize_t *copied)
+static int
+cifs_readdata_to_iov(struct cifs_readdata *rdata, struct iov_iter *iter)
 {
-	int rc = 0;
-	struct iov_iter ii;
-	size_t pos = rdata->offset - offset;
-	ssize_t remaining = rdata->bytes;
-	unsigned char *pdata;
+	size_t remaining = rdata->bytes;
 	unsigned int i;
 
-	/* set up iov_iter and advance to the correct offset */
-	iov_iter_init(&ii, iov, nr_segs, iov_length(iov, nr_segs), 0);
-	iov_iter_advance(&ii, pos);
-
-	*copied = 0;
 	for (i = 0; i < rdata->nr_pages; i++) {
-		ssize_t copy;
 		struct page *page = rdata->pages[i];
-
-		/* copy a whole page or whatever's left */
-		copy = min_t(ssize_t, remaining, PAGE_SIZE);
-
-		/* ...but limit it to whatever space is left in the iov */
-		copy = min_t(ssize_t, copy, iov_iter_count(&ii));
-
-		/* go while there's data to be copied and no errors */
-		if (copy && !rc) {
-			pdata = kmap(page);
-			rc = memcpy_toiovecend(ii.iov, pdata, ii.iov_offset,
-						(int)copy);
-			kunmap(page);
-			if (!rc) {
-				*copied += copy;
-				remaining -= copy;
-				iov_iter_advance(&ii, copy);
-			}
-		}
+		size_t copy = min(remaining, PAGE_SIZE);
+		size_t written = copy_page_to_iter(page, 0, copy, iter);
+		remaining -= written;
+		if (written < copy && iov_iter_count(iter) > 0)
+			break;
 	}
-
-	return rc;
+	return remaining ? -EFAULT : 0;
 }
 
 static void
@@ -2851,6 +2822,7 @@ cifs_iovec_read(struct file *file, const struct iovec *iov,
 	struct cifsFileInfo *open_file;
 	struct cifs_readdata *rdata, *tmp;
 	struct list_head rdata_list;
+	struct iov_iter to;
 	pid_t pid;
 
 	if (!nr_segs)
@@ -2860,6 +2832,8 @@ cifs_iovec_read(struct file *file, const struct iovec *iov,
 	if (!len)
 		return 0;
 
+	iov_iter_init(&to, iov, nr_segs, len, 0);
+
 	INIT_LIST_HEAD(&rdata_list);
 	cifs_sb = CIFS_SB(file->f_path.dentry->d_sb);
 	open_file = file->private_data;
@@ -2917,12 +2891,11 @@ cifs_iovec_read(struct file *file, const struct iovec *iov,
 	if (!list_empty(&rdata_list))
 		rc = 0;
 
+	len = iov_iter_count(&to);
 	/* the loop below should proceed in the order of increasing offsets */
 	list_for_each_entry_safe(rdata, tmp, &rdata_list, list) {
 	again:
 		if (!rc) {
-			ssize_t copied;
-
 			/* FIXME: freezable sleep too? */
 			rc = wait_for_completion_killable(&rdata->done);
 			if (rc)
@@ -2935,10 +2908,7 @@ cifs_iovec_read(struct file *file, const struct iovec *iov,
 					goto again;
 				}
 			} else {
-				rc = cifs_readdata_to_iov(rdata, iov,
-							nr_segs, *poffset,
-							&copied);
-				total_read += copied;
+				rc = cifs_readdata_to_iov(rdata, &to);
 			}
 
 		}
@@ -2946,6 +2916,8 @@ cifs_iovec_read(struct file *file, const struct iovec *iov,
 		kref_put(&rdata->refcount, cifs_uncached_readdata_release);
 	}
 
+	total_read = len - iov_iter_count(&to);
+
 	cifs_stats_bytes_read(tcon, total_read);
 	*poffset += total_read;