ext4: fix partial page writes

While running extended fsx tests to verify the preceeding patches,
a similar bug was also found in the write operation

When ever a write operation begins or ends in a hole,
or extends EOF, the partial page contained in the hole
or beyond EOF needs to be zeroed out.

To correct this the new ext4_discard_partial_page_buffers_no_lock
routine is used to zero out the partial page, but only for buffer
heads that are already unmapped.

Signed-off-by: Allison Henderson <achender@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/inode.c

@@ -2255,6 +2255,7 @@ static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
 	pgoff_t index;
 	struct inode *inode = mapping->host;
 	handle_t *handle;
+	loff_t page_len;
 
 	index = pos >> PAGE_CACHE_SHIFT;
 
@@ -2301,6 +2302,13 @@ retry:
 		 */
 		if (pos + len > inode->i_size)
 			ext4_truncate_failed_write(inode);
+	} else {
+		page_len = pos & (PAGE_CACHE_SIZE - 1);
+		if (page_len > 0) {
+			ret = ext4_discard_partial_page_buffers_no_lock(handle,
+				inode, page, pos - page_len, page_len,
+				EXT4_DISCARD_PARTIAL_PG_ZERO_UNMAPPED);
+		}
 	}
 
 	if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
@@ -2343,6 +2351,7 @@ static int ext4_da_write_end(struct file *file,
 	loff_t new_i_size;
 	unsigned long start, end;
 	int write_mode = (int)(unsigned long)fsdata;
+	loff_t page_len;
 
 	if (write_mode == FALL_BACK_TO_NONDELALLOC) {
 		if (ext4_should_order_data(inode)) {
@@ -2391,6 +2400,16 @@ static int ext4_da_write_end(struct file *file,
 	}
 	ret2 = generic_write_end(file, mapping, pos, len, copied,
 							page, fsdata);
+
+	page_len = PAGE_CACHE_SIZE -
+			((pos + copied - 1) & (PAGE_CACHE_SIZE - 1));
+
+	if (page_len > 0) {
+		ret = ext4_discard_partial_page_buffers_no_lock(handle,
+			inode, page, pos + copied - 1, page_len,
+			EXT4_DISCARD_PARTIAL_PG_ZERO_UNMAPPED);
+	}
+
 	copied = ret2;
 	if (ret2 < 0)
 		ret = ret2;