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

[Waiman Long]

On 04/15/2016 04:17 AM, Dave Chinner wrote:
> On Thu, Apr 14, 2016 at 12:21:13PM -0400, Waiman Long wrote:
> > On 04/13/2016 11:16 PM, Dave Chinner wrote:
> > > On Tue, Apr 12, 2016 at 02:12:54PM -0400, Waiman Long wrote:
> > > > When performing direct I/O, the current ext4 code does
> > > > not pass in the DIO_SKIP_DIO_COUNT flag to dax_do_io() or
> > > > __blockdev_direct_IO() when inode_dio_begin() has, in fact, been
> > > > called. This causes dax_do_io()/__blockdev_direct_IO() to invoke
> > > > inode_dio_begin()/inode_dio_end() internally.  This doubling of
> > > > inode_dio_begin()/inode_dio_end() calls are wasteful.
> > > >
> > > > This patch removes the extra internal inode_dio_begin()/inode_dio_end()
> > > > calls when those calls are being issued by the caller directly. For
> > > > really fast storage systems like NVDIMM, the removal of the extra
> > > > inode_dio_begin()/inode_dio_end() can give a meaningful boost to
> > > > I/O performance.
> > > Doesn't this break truncate IO serialisation?
> > >
> > > i.e. it appears to me that the ext4 use of inode_dio_begin()/
> > > inode_dio_end() does not cover AIO, where the IO is still in flight
> > > when submission returns. i.e. the inode_dio_end() call
> > > needs to be in IO completion, not in the submitter context. The only
> > > reason it doesn't break right now is that the duplicate accounting
> > > in the DIO code is correct w.r.t. AIO. Hence bypassing the DIO
> > > accounting will cause AIO writes to race with truncate.
> > >
> > > Same AIO vs truncate problem occurs with the indirect read case you
> > > modified to skip the direct IO layer accounting.
> > I don't quite understand how the duplicate accounting is correct wrt
> > AIO. Both the direct and indirect paths are something like:
> >
> >      inode_dio_begin()
> >      ...
> >          inode_dio_begin()
> >          ...
> >          inode_dio_end()
> >      ...
> >      inode_dio_end()
> With AIO:
>
> 	inode_dio_begin()
> 	...
> 		inode_dio_begin()
> 		<submit IO, no wait>
> 	...
> 	inode_dio_end()
> <ext4 returns to userspace with AIO+DIO in progress>
>
> <some time later DIO completes>
> 	dio_complete
> 		  inode_dio_end()
>
> IOWs, the ext4 accounting is broken w.r.t. AIO, where IO submission
> does not wait for IO completion before returning.
>
> > 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().

Cheers,
Longman