[Ocfs2-devel] [PATCH] Zero from EOF instead of next block

[Joel Becker <jlbec@evilplan.org>]

On Mon, Feb 14, 2011 at 12:24:01PM -0600, Goldwyn Rodrigues wrote:
> On Mon, Feb 14, 2011 at 1:25 AM, Joel Becker <jlbec@evilplan.org> wrote:
> Why is zeroing to EOC a mistake?

	Because Linux writeback trims all writes to EOB for the block
surrounding i_size.  So when you fill pages to EOC, and blocksize !=
clustersize, you end up with pages that are never flushed to disk.
This becomes a problem.  In reflink CoW, it ends up being a BUG().
	See
http://kerneltrap.org/mailarchive/linux-kernel/2010/6/28/4587750 for the
entire discussion.  The final fix patches ended going in as:

a4bfb4c ocfs2: When zero extending, do it by page.
5693486 ocfs2: Zero the tail cluster when extending past i_size.
693c241 ocfs2: No need to zero pages past i_size.

> I ran my tests again and found the problem to be from EOB to
> End-of-cluster, and of course this happens only with
> blocksize<clustersize. So, in ocfs2_zero_extend rewriting zero_to_size
> to end-of-cluster works correctly  -

	I'm going to check out your test case, but please explain what
you think is broken.

Joel

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