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

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Date:   Tue, 25 Feb 2020 20:55:31 -0600
From:   Goldwyn Rodrigues <rgoldwyn@...e.de>
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.
> 
> 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.
> 
> > 
> > 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?
> 
> We'd need something there iff we care about a good number of written
> in case of the error.  Goldwyn, can you explain what you need this
> number for in btrfs?  Maybe with a pointer to the current code base?

btrfs needs to account for the bytes "processed", failed or
uptodate. This is currently performed in
fs/btrfs/inode.c:__end_write_update_ordered().

For the current development version, how I am using it is in my git
branch btrfs-iomap-dio [1]. The related commit besides this patch
is:

9aeb2b31d10b ("btrfs: Use ->iomap_end() instead of btrfs_dio_data")

[1] https://github.com/goldwynr/linux/tree/btrfs-iomap-dio

-- 
Goldwyn