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 07:35:00 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Josef Bacik <josef@...icpanda.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Goldwyn Rodrigues <rgoldwyn@...e.de>,
        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 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..