linux-ext4 - ext4 stable page writes question

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Message-ID: <87txnf4yeq.fsf@openvz.org>
Date:	Tue, 09 Apr 2013 17:38:21 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	ext4 development <linux-ext4@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, Theodore Ts'o <tytso@....edu>
Subject: ext4 stable page writes question

According to stable write assumptions (1d1d1a767206fb)
grab_cache_page_write_begin() now calls relaxed method wait_for_stable_page()
which will wait for writeback to finish only if bdi demand that.
Commit message states that ext4 may not wait
But there are a lot of write-paths where we expect that:
BUG_ON(!PageLocked(page));
BUG_ON(PageWriteback(page));
And the only reason we avoid this bugon is because of commit 47564bfb95b
which use following trick to avoid lock inversion over journal_start:
page = grab_cache_page_write_begin()
unlock_page(page);
ext4_journal_start()
lock_page(page);
wait_on_page_writeback(page); <<<< unconditional wait

So as far as I understand this was done just by occasion because
ext4_page_mkwrite() use wait_for_stable_page().

So here is my question:  Do we have to wait for page's writeback to
finish for all write paths in ext4 code or we may use
wait_for_stable_page() and should cleanup all places where 
we may trigger BUG_ON(PageWriteback(page));

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