From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Moyer Subject: Re: [PATCH] jbd/2[stable only]: Use WRITE_SYNC_PLUG in journal_commit_transaction. Date: Thu, 14 Jul 2011 16:08:24 -0400 Message-ID: References: <1310467431-23108-1-git-send-email-tm@tao.ma> <20110712123041.GC1293@redhat.com> <4E1C65EA.5060009@tao.ma> <20110714194657.GA16415@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Tao Ma , Vivek Goyal , linux-ext4@vger.kernel.org, stable@kernel.org, Corrado Zoccolo , Jens Axboe To: Jan Kara Return-path: Received: from mx1.redhat.com ([209.132.183.28]:6732 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753065Ab1GNUIp (ORCPT ); Thu, 14 Jul 2011 16:08:45 -0400 In-Reply-To: <20110714194657.GA16415@quack.suse.cz> (Jan Kara's message of "Thu, 14 Jul 2011 21:46:57 +0200") Sender: linux-ext4-owner@vger.kernel.org List-ID: Jan Kara writes: > On Thu 14-07-11 12:30:32, Jeff Moyer wrote: >> Tao Ma writes: >> >> - WRITE_SYNC_PLUG will plug the queue and expects explicity unplug. Who >> >> is doing unplug in this case? >> > See the comments I removed, "we rely on sync_buffer() doing the unplug >> > for us". I removed them cause we all use pluged write now. >> >> Your logic is upside-down. The code currently only uses the _PLUG >> variant when t_synchronous_commit is set, meaning somebody *will* call >> sync_buffer. Simply setting WRITE_SYNC_PLUG doens't mean the upper >> layer is going to issue the unplug. Of course, I'm not 100% sure of the >> journaling process, so it may very well be that there always is an >> unplug. Can Jan or someone comment on that? Anyway, you could test >> this theory by seeing if your kernel generates any timer unplugs in the >> blktrace output. > So I'm not expert in plugging code but from what I understand when we do > wait_on_buffer() (which calls io_schedule()) which will do > blk_flush_plug()), the queue will get unplugged and IO starts. And we wait > for all buffers we submit so we are guaranteed wait_on_buffer() will be > called... Sorry, I should have been more specific. As Vivek mentioned, we're talking about older kernels (pre the blk plugging series). So, the question is, if journal_commit_transaction is called with t_synchronous_commit not set, will the underlying device ever be unplugged by the journal code? My guess is there's no explicit unplug, so it's not correct to replace a WRITE_SYNC with a WRITE_SYNC_PLUG. Cheers, Jeff