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

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Date:   Fri, 28 Feb 2020 13:44:01 -0600
From:   Goldwyn Rodrigues <rgoldwyn@...e.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Ritesh Harjani <riteshh@...ux.ibm.com>, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, darrick.wong@...cle.com
Subject: Re: [PATCH v2] iomap: return partial I/O count on error in
 iomap_dio_bio_actor

On 12:53 25/02, Christoph Hellwig wrote:
> On Fri, Feb 21, 2020 at 10:21:04AM +0530, Ritesh Harjani wrote:
> > >   		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.
> 


Reading the code again, there are a few clarifications.

If iomap_end() handles (written < length) as an error, iomap_apply()
will return an error immediately. It will not execute the 
loop a second time.

On the other hand, if there is no ->iomap_end() defined by the
filesystem such as in the case of XFS, we will need to check for
dio->error in the do {} while loop of iomap_dio_rw().

> In addition to that copied is also iov_iter_reexpand call.  We don't
> really need the re-expand in case of errors, and in fact we also
> have the iov_iter_revert call before jumping out, so this will
> need a little bit more of an audit and properly documented in the
> commit log.

We are still handling this as an error, so why are we concerned about
expanding? There is no success/written returned in iomap_dio_rw() call
in case of an error.

Attached is an updated patch.


-- 
Goldwyn

View attachment "0004-iomap-return-partial-I-O-count-on-error-in-iomap_dio.patch" of type "text/x-patch" (2134 bytes)