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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