Re: [PATCH v3 1/2] ext4: Pass in DIO_SKIP_DIO_COUNT flag if inode_dio_begin() called

[Waiman Long]

On 04/15/2016 06:19 PM, Dave Chinner wrote:
> On Fri, Apr 15, 2016 at 01:17:41PM -0400, Waiman Long wrote:
> > On 04/15/2016 04:17 AM, Dave Chinner wrote:
> > > On Thu, Apr 14, 2016 at 12:21:13PM -0400, Waiman Long wrote:
> > > > What the patch does is to eliminate the innermost
> > > > inode_dio_begin/end pair.
> > > Yes, and with that change inode_dio_wait() no longer waits for
> > > AIO+DIO writes on ext4, hence breaking truncate IO barrier
> > > requirements of inode_dio_wait().
> > >
> > > Cheers,
> > >
> > > Dave.
> > You are right and thank for pointing this out to me. I think I focus too
> > much on the dax_do_io() internal and didn't realize that inode_dio_end() can
> > be deferred in __blockdev_direct_IO(). I will update my patch to eliminate
> > the extra inode_dio_begin/end pair only for dax_do_io().
> Even there there is the risk that a future change will break ext4.
> the ext4 code needs fixing first, then you can look at skipping the
> DIO based counting everywhere.
>
> i.e. fix the root cause of the problem, don't hack around it or
> throw band-aids over it.
>
> Cheers,
>
> Dave.

I agree that the ext4 code needs fixing w.r.t. the problem that you 
found. That will take more time and testing. In the mean time, I think 
it is OK to pick the low-hanging fruits that are handled by my patch.

Cheers,
Longman