From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755707AbdJJIpG (ORCPT ); Tue, 10 Oct 2017 04:45:06 -0400 Received: from mx2.suse.de ([195.135.220.15]:52207 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752207AbdJJIpF (ORCPT ); Tue, 10 Oct 2017 04:45:05 -0400 Date: Tue, 10 Oct 2017 10:45:01 +0200 From: Jan Kara To: Andrew Morton Cc: Yafang Shao , jack@suse.cz, mhocko@suse.com, hannes@cmpxchg.org, vdavydov.dev@gmail.com, jlayton@redhat.com, nborisov@suse.com, tytso@mit.edu, mawilcox@microsoft.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jens Axboe Subject: Re: [PATCH] mm/page-writeback.c: fix bug caused by disable periodic writeback Message-ID: <20171010084501.GC775@quack2.suse.cz> References: <1507330684-2205-1-git-send-email-laoar.shao@gmail.com> <20171009154212.bdf3645a2dce5d540657914b@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171009154212.bdf3645a2dce5d540657914b@linux-foundation.org> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 09-10-17 15:42:12, Andrew Morton wrote: > On Sat, 7 Oct 2017 06:58:04 +0800 Yafang Shao wrote: > > > After disable periodic writeback by writing 0 to > > dirty_writeback_centisecs, the handler wb_workfn() will not be > > entered again until the dirty background limit reaches or > > sync syscall is executed or no enough free memory available or > > vmscan is triggered. > > So the periodic writeback can't be enabled by writing a non-zero > > value to dirty_writeback_centisecs > > As it can be disabled by sysctl, it should be able to enable by > > sysctl as well. > > > > ... > > > > --- a/mm/page-writeback.c > > +++ b/mm/page-writeback.c > > @@ -1972,7 +1972,13 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb) > > int dirty_writeback_centisecs_handler(struct ctl_table *table, int write, > > void __user *buffer, size_t *length, loff_t *ppos) > > { > > - proc_dointvec(table, write, buffer, length, ppos); > > + unsigned int old_interval = dirty_writeback_interval; > > + int ret; > > + > > + ret = proc_dointvec(table, write, buffer, length, ppos); > > + if (!ret && !old_interval && dirty_writeback_interval) > > + wakeup_flusher_threads(0, WB_REASON_PERIODIC); > > + > > return 0; > > We could do with a code comment here, explaining why this code exists. > > And... I'm not sure it works correctly? For example, if a device > doesn't presently have bdi_has_dirty_io() then wakeup_flusher_threads() > will skip it and the periodic writeback still won't be started? This works correctly. For this case __mark_inode_dirty() has: if (bdi_cap_writeback_dirty(wb->bdi) && wakeup_bdi) wb_wakeup_delayed(wb); So periodic writeback gets automatically started once first dirty inode appears on a bdi. > (why does the dirty_writeback_interval==0 special case exist, btw? > Seems to be a strange thing to do). I guess to prevent busylooping? But I'm not sure... > (and what happens if the interval was set to 1 hour and the user > rewrites that to 1 second? Does that change take 1 hour to take > effect?) That's a good point I didn't think about. So probably we should do the wakeup whenever dirty_writeback_interval changes. Honza -- Jan Kara SUSE Labs, CR