On Fri, Apr 29, 2016 at 12:38:20PM -0400, Waiman Long wrote:

From my testing, it looked like that parallel overwrites to the same file in
an ext4 filesystem on DAX can happen in parallel even if their range
overlaps. It was mainly because the code will drop the i_mutex before the
write. That means the overlapped blocks can get garbage. I think this is a
problem, but I am not expert in the ext4 filesystem to say for sure. I would
like to know your thought on that.

That's another issue with dax I/O pretending to be direct I/O..  Because
it isn't we'll need to synchronize it like buffered I/O and not like
direct I/O in all file systems.