Re: [PATCH 22/34] xfs: make xfs_writepage_map extent map centric

On Fri, May 25, 2018 at 08:19:00AM +0200, Christoph Hellwig wrote:

On Thu, May 24, 2018 at 02:13:56PM -0400, Brian Foster wrote:

quoted

Ok, so I guess writeback can see uptodate blocks over a hole if some
other block in that page is dirty.

Yes.

quoted

Perhaps we could make sure that a
dirty page has at least one block that maps to an actual extent or
otherwise the page has been truncated..?

We have the following comment near the end of xfs_writepage_map:

That comment is what I'm basing on...

	/*
	 * We can end up here with no error and nothing to write if we
	 * race with a partial page truncate on a sub-page block sized
	 * filesystem. In that case we need to mark the page clean.
	 */

So we can correctly end up with nothing to write on a dirty page, but it
presumes a race with truncate. So suppose we end up with a dirty page,
at least one uptodate block, count is zero (i.e., due to holes) and
i_size is beyond the page. Would that not be completely bogus? If bogus,
I think that would at least detect the dumb example I posted earlier.

Brian

And I'm pretty sure I managed to hit that case easily in xfstests,
as my initial handling of it was wrong.  So I don't think we can
even check for that.

quoted

I guess having another dirty block bitmap similar to
iomap_page->uptodate could be required to tell for sure whether a
particular block should definitely have a block on-disk or not. It may
not be worth doing that just for additional error checks, but I still
have to look into the last few patches to grok all the iomap_page stuff.

I don't think it's worth it.  The sub-page dirty tracking has been one
of the issues with the buffer head code that caused a lot of problems,
and that we want to get rid of.
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