linux-ext4 - Re: [PATCH v2] iomap: return partial I/O count on error in iomap_dio_bio

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Message-ID: <20200221124833.xfwz7gskkcfdkxol@fiona>
Date:   Fri, 21 Feb 2020 06:48:33 -0600
From:   Goldwyn Rodrigues <rgoldwyn@...e.com>
To:     Ritesh Harjani <riteshh@...ux.ibm.com>
Cc:     linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        hch@...radead.org, darrick.wong@...cle.com
Subject: Re: [PATCH v2] iomap: return partial I/O count on error in
 iomap_dio_bio_actor

On 10:21 21/02, Ritesh Harjani wrote:
> 
> 
> On 2/20/20 8:53 PM, 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.
> > 
> > Signed-off-by: Goldwyn Rodrigues <rgoldwyn@...e.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;
> >   		}
> 
> But if I am seeing this correctly, even after there was a dio->error
> if you return copied > 0, then the loop in iomap_dio_rw will continue
> for next iteration as well. Until the second time it won't copy
> anything since dio->error is set and from there I guess it may return
> 0 which will break the loop.
> 
> Is this the correct flow? Shouldn't the while loop doing
> iomap_apply in iomap_dio_rw should also break in case of
> dio->error? Or did I miss anything?
> 

Yes, We can save an extra iteration by checking for dio->error in the
while loop of iomap_dio_rw(). 

-- 
Goldwyn