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

[Waiman Long]

On 04/19/2016 07:01 PM, Dave Chinner wrote:
> On Mon, Apr 18, 2016 at 03:46:46PM -0400, Waiman Long wrote:
> > 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.
> > 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.
> IOWs, you're saying that you won't fix the problem, because all you
> care about is scalability results. This is how we end up with code
> that breaks randomly in future because if it doesn't get fixed now,
> nobody will fix the underlying problem. So, fix it now, fix it
> properly and you still get your scalability improvement without
> leaving a landmine that will explode on someone else in future.
>
> Fix it now, fix it properly.

I am not saying that I will not fix it. I am just saying that I need 
more time to fully understand what code changes need to be done. I am 
not that well versed in the filesystem internal, though it will be a 
good learning experience for me.

Cheers,
Longman