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

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Date:   Fri, 20 Mar 2020 10:35:28 -0500
From:   Goldwyn Rodrigues <rgoldwyn@...e.de>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Josef Bacik <josef@...icpanda.com>, linux-fsdevel@...r.kernel.org,
        riteshh@...ux.ibm.com, linux-ext4@...r.kernel.org,
        darrick.wong@...cle.com, willy@...radead.org,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH v2] iomap: return partial I/O count on error in
 iomap_dio_bio_actor

On  7:35 20/03, Christoph Hellwig wrote:
> On Fri, Mar 20, 2020 at 10:23:43AM -0400, Josef Bacik wrote:
> > I'm not sure what you're looking at specifically wrt error handling, but I
> > can explain __endio_write_update_ordered.
> > 
> > Btrfs has ordered extents to keep track of an extent that currently has IO
> > being done on it.  Generally that IO takes multiple bio's, so we keep track
> > of the outstanding size of the IO being done, and each bio completes and
> > thus removes its size from the pending size.  If any one of those bios has
> > an error we need to make sure we discard the whole ordered extent, as part
> > of it won't be valid. Just a cursory look at the current code I assume
> > that's what's confusing you, we call this when we have an error in the
> > O_DIRECT code.  This is just so we get the proper cleanup for the ordered
> > extent.  People will wait on the ordered extent to be completed, so if we've
> > started an ordered extent and aren't able to complete the range we need to
> > do __endio_write_update_ordered() so that the ordered extent is finished and
> > we wakeup any waiters.
> > 
> > Does this help?  If I need to I can context switch into whatever you're
> > looking at, but I'm going to avoid looking and hope I can just shout useful
> > information in your direction ;).  Thanks,
> 
> Yes, this helps a lot.  This is about the patches from Goldwyn to
> convert btrfs to use the iomap direct I/O code.  And in that series
> he currently calls __endio_write_update_ordered from the ->iomap_end
> method, which for direct I/O is called after all bios are submitted
> to complete ordered extents for a range after an I/O error, that
> is one that no I/O has been submitted to, and the accounting for that
> is a little complicated..

I think you meant "some" instead of "no".

Yes, keeping the information in iomap->private and setting in
btrfs_submit_direct() would be better. I will modify the code and
re-test. Thanks!

-- 
Goldwyn