From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-it0-f71.google.com (mail-it0-f71.google.com [209.85.214.71]) by kanga.kvack.org (Postfix) with ESMTP id BEF5D6B0253 for ; Wed, 11 Oct 2017 00:06:37 -0400 (EDT) Received: by mail-it0-f71.google.com with SMTP id l196so679154itl.15 for ; Tue, 10 Oct 2017 21:06:37 -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 23sor2078263ioc.168.2017.10.10.21.06.36 for (Google Transport Security); Tue, 10 Oct 2017 21:06:36 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20171010093353.GG775@quack2.suse.cz> References: <1507330684-2205-1-git-send-email-laoar.shao@gmail.com> <20171009154212.bdf3645a2dce5d540657914b@linux-foundation.org> <20171010084817.GD775@quack2.suse.cz> <20171010093353.GG775@quack2.suse.cz> From: Yafang Shao Date: Wed, 11 Oct 2017 12:06:35 +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 17:33 GMT+08:00 Jan Kara : > On Tue 10-10-17 17:14:48, Yafang Shao wrote: >> 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. > > Yes, I think we should wakeup for every change of dirty_writeback_interval. > And immediate wakeup is not a problem IMO. > Got it! 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