From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-it0-f70.google.com (mail-it0-f70.google.com [209.85.214.70]) by kanga.kvack.org (Postfix) with ESMTP id 292D06B0268 for ; Tue, 10 Oct 2017 05:14:50 -0400 (EDT) Received: by mail-it0-f70.google.com with SMTP id 186so843157itu.4 for ; Tue, 10 Oct 2017 02:14:50 -0700 (PDT) Received: from mail-sor-f41.google.com (mail-sor-f41.google.com. [209.85.220.41]) by mx.google.com with SMTPS id p129sor3520505ite.52.2017.10.10.02.14.49 for (Google Transport Security); Tue, 10 Oct 2017 02:14:49 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20171010084817.GD775@quack2.suse.cz> References: <1507330684-2205-1-git-send-email-laoar.shao@gmail.com> <20171009154212.bdf3645a2dce5d540657914b@linux-foundation.org> <20171010084817.GD775@quack2.suse.cz> From: Yafang Shao Date: Tue, 10 Oct 2017 17:14:48 +0800 Message-ID: Subject: Re: [PATCH] mm/page-writeback.c: fix bug caused by disable periodic writeback Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Jan Kara Cc: Andrew Morton , mhocko@suse.com, Johannes Weiner , vdavydov.dev@gmail.com, jlayton@redhat.com, nborisov@suse.com, Theodore Ts'o , mawilcox@microsoft.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jens Axboe 2017-10-10 16:48 GMT+08:00 Jan Kara : > On Tue 10-10-17 16:00:29, Yafang Shao wrote: >> 2017-10-10 6:42 GMT+08:00 Andrew Morton : >> > 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. >> > >> >> OK. I will comment here. >> >> > 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? >> > >> >> That's an issue. >> The periodic writeback won't be started. >> >> Maybe we'd better call wb_wakeup_delayed(wb) here to bypass the >> bdi_has_dirty_io() check ? > > Well, wb_wakeup_delayed() would be more appropriate but you'd then have to > iterate over all bdis and wbs to be able to call it which IMO isn't worth > the pain for a special case like this. But the decision is worth mentioning > in the comment. Also wakeup_flusher_threads() does in principle what you > need - see my reply to Andrew for details. > > Honza Thanks for your explaination. I understood. I will mention it in the comment. Should we do the wakeup whenever dirty_writeback_interval changes ? If we still use wakeup_flusher_threads(), it will wakeup the flusher threads immediately after we make the change. Thanks Yafang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org