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Date:	Mon, 26 Nov 2007 23:17:36 -0500
From:	Erez Zadok <>
To:	Hugh Dickins <>
Cc:	Erez Zadok <>,
Subject: Re: unionfs: several more problems 

In message <>, Hugh Dickins writes:
> On Mon, 26 Nov 2007, Erez Zadok wrote:
> > The small patch below fixed the problem.  Let me know what you think.
> I've one issue with it: please move that wait_on_page_writeback before
> the clear_page_dirty_for_io instead of after it, then resubmit your 14/16.

Done, tested, and working.  Here's the revised patch (pushed to unionfs.git
on korg).



Unionfs: prevent multiple writers to lower_page

Without this patch, the LTP fs test "rwtest04" triggers a
BUG_ON(PageWriteback(page)) in fs/buffer.c:1706.

CC: Hugh Dickins <>

Signed-off-by: Erez Zadok <>
diff --git a/fs/unionfs/mmap.c b/fs/unionfs/mmap.c
index 623a913..74f2e53 100644
--- a/fs/unionfs/mmap.c
+++ b/fs/unionfs/mmap.c
@@ -72,6 +72,7 @@ static int unionfs_writepage(struct page *page, struct writeback_control *wbc)
+	wait_on_page_writeback(lower_page); /* prevent multiple writers */
 	clear_page_dirty_for_io(lower_page); /* emulate VFS behavior */
 	err = lower_mapping->a_ops->writepage(lower_page, wbc);
 	if (err < 0)
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