NFS: Allow nfs_updatepage to extend a write under additional circumstances

Currently nfs_updatepage allows a write to be extended to cover a full
page only if we don't have a byte range lock lock on the file... but if
we have a write delegation on the file or if we have the whole file
locked for writing then we should be allowed to extend the write as
well.
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
[Trond: fix up call to nfs_have_delegation()]
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/write.c

@@ -888,6 +888,28 @@ static bool nfs_write_pageuptodate(struct page *page, struct inode *inode)
 	return PageUptodate(page) != 0;
 }
 
+/* If we know the page is up to date, and we're not using byte range locks (or
+ * if we have the whole file locked for writing), it may be more efficient to
+ * extend the write to cover the entire page in order to avoid fragmentation
+ * inefficiencies.
+ *
+ * If the file is opened for synchronous writes or if we have a write delegation
+ * from the server then we can just skip the rest of the checks.
+ */
+static int nfs_can_extend_write(struct file *file, struct page *page, struct inode *inode)
+{
+	if (file->f_flags & O_DSYNC)
+		return 0;
+	if (NFS_PROTO(inode)->have_delegation(inode, FMODE_WRITE))
+		return 1;
+	if (nfs_write_pageuptodate(page, inode) && (inode->i_flock == NULL ||
+			(inode->i_flock->fl_start == 0 &&
+			inode->i_flock->fl_end == OFFSET_MAX &&
+			inode->i_flock->fl_type != F_RDLCK)))
+		return 1;
+	return 0;
+}
+
 /*
  * Update and possibly write a cached page of an NFS file.
  *
@@ -908,14 +930,7 @@ int nfs_updatepage(struct file *file, struct page *page,
 		file->f_path.dentry->d_name.name, count,
 		(long long)(page_file_offset(page) + offset));
 
-	/* If we're not using byte range locks, and we know the page
-	 * is up to date, it may be more efficient to extend the write
-	 * to cover the entire page in order to avoid fragmentation
-	 * inefficiencies.
-	 */
-	if (nfs_write_pageuptodate(page, inode) &&
-			inode->i_flock == NULL &&
-			!(file->f_flags & O_DSYNC)) {
+	if (nfs_can_extend_write(file, page, inode)) {
 		count = max(count + offset, nfs_page_length(page));
 		offset = 0;
 	}