ecryptfs: remove unnecessary decrypt when extending a file

Removes an unecessary page decrypt from ecryptfs_begin_write when the
page is beyond the current file size. Previously, the call to
ecryptfs_decrypt_page would result in a read of 0 bytes, but still
attempt to decrypt an entire page. This patch detects that case and
merely zeros the page before marking it up-to-date.
Signed-off-by: Frank Swiderski <fes@chromium.org>
Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>

fs/ecryptfs/mmap.c

@@ -290,6 +290,7 @@ static int ecryptfs_write_begin(struct file *file,
 		return -ENOMEM;
 	*pagep = page;
 
+	prev_page_end_size = ((loff_t)index << PAGE_CACHE_SHIFT);
 	if (!PageUptodate(page)) {
 		struct ecryptfs_crypt_stat *crypt_stat =
 			&ecryptfs_inode_to_private(mapping->host)->crypt_stat;
@@ -334,19 +335,24 @@ static int ecryptfs_write_begin(struct file *file,
 				}
 				SetPageUptodate(page);
 			}
+		} else {
+			if (prev_page_end_size
+			    >= i_size_read(page->mapping->host)) {
+				zero_user(page, 0, PAGE_CACHE_SIZE);
 			} else {
 				rc = ecryptfs_decrypt_page(page);
 				if (rc) {
-				printk(KERN_ERR "%s: Error decrypting page "
-				       "at index [%ld]; rc = [%d]\n",
+					printk(KERN_ERR "%s: Error decrypting "
+					       "page at index [%ld]; "
+					       "rc = [%d]\n",
 					       __func__, page->index, rc);
 					ClearPageUptodate(page);
 					goto out;
 				}
+			}
 			SetPageUptodate(page);
 		}
 	}
-	prev_page_end_size = ((loff_t)index << PAGE_CACHE_SHIFT);
 	/* If creating a page or more of holes, zero them out via truncate.
 	 * Note, this will increase i_size. */
 	if (index != 0) {