linux-kernel - Re: unionfs: several more problems

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Message-Id: <200711261653.lAQGr2Et011553@agora.fsl.cs.sunysb.edu>
Date:	Mon, 26 Nov 2007 11:53:02 -0500
From:	Erez Zadok <ezk@...sunysb.edu>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Erez Zadok <ezk@...sunysb.edu>, linux-kernel@...r.kernel.org
Subject: Re: unionfs: several more problems 

In message <Pine.LNX.4.64.0711201802480.9934@...nde.wat.veritas.com>, Hugh Dickins writes:

[...]
> Please try running LTP (e.g. ltp-full-20071031.tgz) after something like
[...]

Hugh,

I just posted a series of patches to unionfs (already in unionfs.git on
korg), which fix every problem LTP found, as well as other problems
mentioned in your email.  With this series of patches, the same set of tests
which pass on ext3 also pass with unionfs mounted over ext3.

I'd like to call your attention to one small unionfs_writepage fix.  One of
the ltp fs tests (rwtest04) triggered a BUG_ON(PageWriteback(page)) in
fs/buffer.c:1706, when we call the lower ->writepage.  This was due to
multiple writers to the same page.  Looking at other kernel code, it seems
that the right fix is to use wait_on_page_writeback(lower_page) to serialize
concurrent writebacks to the same page, no?  The small patch below fixed the
problem.  Let me know what you think.

Thanks,
Erez.

diff --git a/fs/unionfs/mmap.c b/fs/unionfs/mmap.c
index eef43aa..d0fd4d0 100644
--- a/fs/unionfs/mmap.c
+++ b/fs/unionfs/mmap.c
@@ -73,6 +73,7 @@ static int unionfs_writepage(struct page *page, struct writeback_control *wbc)

 	BUG_ON(!lower_mapping->a_ops->writepage);
 	clear_page_dirty_for_io(lower_page); /* emulate VFS behavior */
+	wait_on_page_writeback(lower_page); /* prevent multiple writers */
 	err = lower_mapping->a_ops->writepage(lower_page, wbc);
 	if (err < 0)
 		goto out_release;
-
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