Re: [PATCH v2] iomap: return partial I/O count on error in iomap_dio_bio_actor

From: Goldwyn Rodrigues <rgoldwyn@suse.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	hch@infradead.org, darrick.wong@oracle.com
Subject: Re: [PATCH v2] iomap: return partial I/O count on error in iomap_dio_bio_actor
Date: Thu, 20 Feb 2020 20:06:53 -0600	[thread overview]
Message-ID: <20200221020653.ontjbxhorjymsbqx@fiona> (raw)
In-Reply-To: <20200220174255.GA24185@bombadil.infradead.org>

On  9:42 20/02, Matthew Wilcox wrote:
> On Thu, Feb 20, 2020 at 09:23:55AM -0600, Goldwyn Rodrigues wrote:
> > In case of a block device error, written parameter in iomap_end()
> > is zero as opposed to the amount of submitted I/O.
> > Filesystems such as btrfs need to account for the I/O in ordered
> > extents, even if it resulted in an error. Having (incomplete)
> > submitted bytes in written gives the filesystem the amount of data
> > which has been submitted before the error occurred, and the
> > filesystem code can choose how to use it.
> > 
> > The final returned error for iomap_dio_rw() is set by
> > iomap_dio_complete().
> > 
> > Partial writes in direct I/O are considered an error. So,
> > ->iomap_end() using written == 0 as error must be changed
> > to written < length. In this case, ext4 is the only user.
> 
> I really had a hard time understanding this.  I think what you meant
> was:
> 
> Currently, I/Os that complete with an error indicate this by passing
> written == 0 to the iomap_end function.  However, btrfs needs to know how
> many bytes were written for its own accounting.  Change the convention
> to pass the number of bytes which were actually written, and change the
> only user to check for a short write instead of a zero length write.

Yes, thats right.  I was trying to cover base from the previous patch
and made a mess of it all. Thanks..

> 
> > Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> > ---
> >  fs/ext4/inode.c      | 2 +-
> >  fs/iomap/direct-io.c | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index e60aca791d3f..e50e7414351a 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -3475,7 +3475,7 @@ static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
> >  	 * the I/O. Any blocks that may have been allocated in preparation for
> >  	 * the direct I/O will be reused during buffered I/O.
> >  	 */
> > -	if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written == 0)
> > +	if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written < length)
> >  		return -ENOTBLK;
> >  
> >  	return 0;
> > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > index 41c1e7c20a1f..01865db1bd09 100644
> > --- a/fs/iomap/direct-io.c
> > +++ b/fs/iomap/direct-io.c
> > @@ -264,7 +264,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
> >  		size_t n;
> >  		if (dio->error) {
> >  			iov_iter_revert(dio->submit.iter, copied);
> > -			copied = ret = 0;
> > +			ret = 0;
> >  			goto out;
> >  		}
> >  
> > -- 
> > 2.25.0
> > 

-- 
Goldwyn