[PATCH v2 2/5] ext4: prevent data corruption with journaling + DAX

[Ross Zwisler]

The current code has the potential for data corruption when changing an
inode's journaling mode, as that can result in a subsequent unsafe change
in S_DAX.

I've captured an instance of this data corruption in the following fstest:

https://patchwork.kernel.org/patch/9948377/

Prevent this data corruption from happening by disallowing changes to the
journaling mode if the '-o dax' mount option was used.  This means that for
a given filesystem we could have a mix of inodes using either DAX or
data journaling, but whatever state the inodes are in will be held for the
duration of the mount.

Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Suggested-by: Jan Kara <jack@xxxxxxx>
---
 fs/ext4/inode.c |  5 -----
 fs/ext4/ioctl.c | 16 +++++++++++++---
 2 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index e963508..3207333 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5971,11 +5971,6 @@ int ext4_change_inode_journal_flag(struct inode *inode, int val)
 		ext4_clear_inode_flag(inode, EXT4_INODE_JOURNAL_DATA);
 	}
 	ext4_set_aops(inode);
-	/*
-	 * Update inode->i_flags after EXT4_INODE_JOURNAL_DATA was updated.
-	 * E.g. S_DAX may get cleared / set.
-	 */
-	ext4_set_inode_flags(inode);
 
 	jbd2_journal_unlock_updates(journal);
 	percpu_up_write(&sbi->s_journal_flag_rwsem);
diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index afb66d4..b0b754b 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -290,10 +290,20 @@ static int ext4_ioctl_setflags(struct inode *inode,
 	if (err)
 		goto flags_out;
 
-	if ((jflag ^ oldflags) & (EXT4_JOURNAL_DATA_FL))
+	if ((jflag ^ oldflags) & (EXT4_JOURNAL_DATA_FL)) {
+		/*
+		 * Changes to the journaling mode can cause unsafe changes to
+		 * S_DAX if we are using the DAX mount option.
+		 */
+		if (test_opt(inode->i_sb, DAX)) {
+			err = -EBUSY;
+			goto flags_out;
+		}
+
 		err = ext4_change_inode_journal_flag(inode, jflag);
-	if (err)
-		goto flags_out;
+		if (err)
+			goto flags_out;
+	}
 	if (migrate) {
 		if (flags & EXT4_EXTENTS_FL)
 			err = ext4_ext_migrate(inode);
-- 
2.9.5