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

[Waiman Long]

On 05/01/2016 01:28 PM, Christoph Hellwig wrote:
> 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.

From what I saw in the code, I think filemap_write_and_wait_range()
should have prevented concurrent overwrites from stepping on each
other for non-DAX I/O.  However it is essentially a no-op for DAX
I/O and so the protection is gone.

I am planning to send out a patch to disable mutex dropping for DAX
overwrite. There is still an issue on the read side. If journal is
disabled and the dioread_nolock mount option is used, read will done
without locking. Again, the filemap_write_and_wait_range() check on
the read side will not protect against write.

Cheers,
Longman