From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kara Subject: Re: ext4 stable page writes question Date: Tue, 9 Apr 2013 16:28:05 +0200 Message-ID: <20130409142805.GF13672@quack.suse.cz> References: <87txnf4yeq.fsf@openvz.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: ext4 development , Jan Kara , Theodore Ts'o To: Dmitry Monakhov Return-path: Received: from cantor2.suse.de ([195.135.220.15]:59086 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932929Ab3DIO2H (ORCPT ); Tue, 9 Apr 2013 10:28:07 -0400 Content-Disposition: inline In-Reply-To: <87txnf4yeq.fsf@openvz.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Tue 09-04-13 17:38:21, Dmitry Monakhov wrote: > 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. Yes. > 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)); Really? The only places I can find are in writeback path and there we have wait_on_page_writeback() (either in write_cache_pages() or in ext4_da_writepages()). > 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 No, I think this is really independent. ext4 should be fine when write & writeback are running in parallel for a page. > 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)); If there's any such place, please tell me how we could trigger it... Honza -- Jan Kara SUSE Labs, CR