ext4: Fix ext4_should_writeback_data() for no-journal mode

ext4_should_writeback_data() had an incorrect sequence of
tests to determine if it should return 0 or 1: in
particular, even in no-journal mode, 0 was being returned
for a non-regular-file inode.

This meant that, in non-journal mode, we would use
ext4_journalled_aops for directories, symlinks, and other
non-regular files.  However, calling journalled aop
callbacks when there is no valid handle, can cause problems.

This would cause a kernel crash with Jan Kara's commit
2d859db3 ("ext4: fix data corruption in inodes with
journalled data"), because we now dereference 'handle' in
ext4_journalled_write_end().

I also added BUG_ONs to check for a valid handle in the
obviously journal-only aops callbacks.

I tested this running xfstests with a scratch device in
these modes:

   - no-journal
   - data=ordered
   - data=writeback
   - data=journal

All work fine; the data=journal run has many failures and a
crash in xfstests 074, but this is no different from a
vanilla kernel.
Signed-off-by: NCurt Wohlgemuth <curtw@google.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/ext4_jbd2.h

@@ -289,10 +289,10 @@ static inline int ext4_should_order_data(struct inode *inode)
 
 static inline int ext4_should_writeback_data(struct inode *inode)
 {
-	if (!S_ISREG(inode->i_mode))
-		return 0;
 	if (EXT4_JOURNAL(inode) == NULL)
 		return 1;
+	if (!S_ISREG(inode->i_mode))
+		return 0;
 	if (ext4_test_inode_flag(inode, EXT4_INODE_JOURNAL_DATA))
 		return 0;
 	if (test_opt(inode->i_sb, DATA_FLAGS) == EXT4_MOUNT_WRITEBACK_DATA)

fs/ext4/inode.c

@@ -983,6 +983,8 @@ static int ext4_journalled_write_end(struct file *file,
 	from = pos & (PAGE_CACHE_SIZE - 1);
 	to = from + len;
 
+	BUG_ON(!ext4_handle_valid(handle));
+
 	if (copied < len) {
 		if (!PageUptodate(page))
 			copied = 0;
@@ -1699,6 +1701,8 @@ static int __ext4_journalled_writepage(struct page *page,
 		goto out;
 	}
 
+	BUG_ON(!ext4_handle_valid(handle));
+
 	ret = walk_page_buffers(handle, page_bufs, 0, len, NULL,
 				do_journal_get_write_access);