linux-ext4 - notes in the WARN_ON in ext4_release

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Date:	Sat, 29 Dec 2012 02:58:54 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	linux-ext4@...r.kernel.org
Subject: notes in the WARN_ON in ext4_release_page() with data=journal

I've been trying to track down with a WARN_ON in ext4_release_page()
which can be reproduced by running xfstests #247 in data=journalled
mode.  I'm sending the results of this investigation to linux-ext4
partially as a note to myself, or in case other people notice it.

What's going on is while the O_DIRECT write is happening, in
generic_file_direct_write(), first we force all of the pages in question
to disk using filemap_write_and_wait_range().  Then, we invalidate all
of the pages in question.  If there is a process which is also modifying
the page using mmap writes (which is the case in test #247), the page
can get marked dirty after its writeback, and before the pages are
released by invalidate_inode_pages2_range().

(The PageChecked flag is set by ext4_journalled_set_page_dirty(), and
cleared by __ext4_journalled_writepage() --- for an explanation why,
please see comments for __ext4_journalled_writepage().  When the page
gets released, if ext4_release_page() notices that the PageChecked flag
is still set, it will trigger the WARN_ON.)

This WARN_ON(), then since the stack trace involves
invalidate_page_range(), is harmless.  If the user is doing unaligned
DIO writes, the changes made by the mmap might disappear, and which
means our cache coherency guarantees made by our O_DIRECT guarantee is
not absolute.  I'm not really worried about it since, if an application
is racing an mmap modification with an O_DIRECT write, it's just asking
to lose --- and this race has been there for a long time.  It's just
that the data=journal machinery makes this noticeable.

                                           - Ted
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