From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dmitry Monakhov Subject: Re: ext4 stable page writes question Date: Tue, 09 Apr 2013 19:01:32 +0400 Message-ID: <87r4ijrbn7.fsf@openvz.org> References: <87txnf4yeq.fsf@openvz.org> <20130409142805.GF13672@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: ext4 development , Jan Kara , Theodore Ts'o To: Jan Kara Return-path: Received: from mail-lb0-f170.google.com ([209.85.217.170]:40886 "EHLO mail-lb0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964938Ab3DIPBh (ORCPT ); Tue, 9 Apr 2013 11:01:37 -0400 Received: by mail-lb0-f170.google.com with SMTP id x11so6970765lbi.29 for ; Tue, 09 Apr 2013 08:01:35 -0700 (PDT) In-Reply-To: <20130409142805.GF13672@quack.suse.cz> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Tue, 9 Apr 2013 16:28:05 +0200, Jan Kara wrote: > 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... move_extent.c:mext_page_mkuptodate() expect that. IMHO it is reasonable to force behavior here even if we may not do that on other places because defragmentation code should be 101% reliable and performance is not important here. > > Honza > -- > Jan Kara > SUSE Labs, CR