Re: [PATCH v5 0/2] ext4: Improve parallel I/O performance on NVDIMM

From: Christoph Hellwig
Date: Sun May 01 2016 - 13:29:13 EST

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.