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

[Waiman Long]

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()

What the patch does is to eliminate the innermost inode_dio_begin/end 
pair. Unless there is a difference between a dio count of 2 vs. 1, I 
can't see how the code correctness differ with and without my patch.

Cheers,
Longman