linux-kernel - Re: [PATCH 07/13] xfs: allow merging ioends over append boundaries

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Date:   Fri, 28 Jun 2019 07:51:43 +0200
From:   Christoph Hellwig <hch@....de>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Christoph Hellwig <hch@....de>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/13] xfs: allow merging ioends over append boundaries

On Thu, Jun 27, 2019 at 11:23:09AM -0700, Darrick J. Wong wrote:
> On Thu, Jun 27, 2019 at 12:48:30PM +0200, Christoph Hellwig wrote:
> > There is no real problem merging ioends that go beyond i_size into an
> > ioend that doesn't.  We just need to move the append transaction to the
> > base ioend.  Also use the opportunity to use a real error code instead
> > of the magic 1 to cancel the transactions, and write a comment
> > explaining the scheme.
> > 
> > Signed-off-by: Christoph Hellwig <hch@....de>
> 
> Reading through this patch, I have a feeling it fixes the crash that
> Zorro has been seeing occasionally with generic/475...

So you think for some reason the disk i_size changes underneath and thus
the xfs_ioend_is_append misfired vs the actual transaction allocations?
I didn't even think of that, but using the different checks sure sounds
dangerous.  So yes, we'd either need to backport my patch, or at least
replace the checks in xfs_ioend_can_merge with direct checks of
io_append_trans.