From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f197.google.com (mail-pf0-f197.google.com [209.85.192.197]) by kanga.kvack.org (Postfix) with ESMTP id 2FA166B0038 for ; Tue, 26 Sep 2017 07:27:02 -0400 (EDT) Received: by mail-pf0-f197.google.com with SMTP id q75so17912971pfl.1 for ; Tue, 26 Sep 2017 04:27:02 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id v5si5785019pgr.387.2017.09.26.04.27.00 for (version=TLS1 cipher=AES128-SHA bits=128/128); Tue, 26 Sep 2017 04:27:01 -0700 (PDT) Date: Tue, 26 Sep 2017 13:26:56 +0200 From: Michal Hocko Subject: Re: [PATCH v3] mm: introduce validity check on vm dirtiness settings Message-ID: <20170926112656.tbu7nr2lxdqt5rft@dhcp22.suse.cz> References: <1505861015-11919-1-git-send-email-laoar.shao@gmail.com> <20170926102532.culqxb45xwzafomj@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Yafang Shao Cc: Jan Kara , akpm@linux-foundation.org, 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 On Tue 26-09-17 19:06:37, Yafang Shao wrote: > 2017-09-26 18:25 GMT+08:00 Michal Hocko : > > On Wed 20-09-17 06:43:35, Yafang Shao wrote: > >> we can find the logic in domain_dirty_limits() that > >> when dirty bg_thresh is bigger than dirty thresh, > >> bg_thresh will be set as thresh * 1 / 2. > >> if (bg_thresh >= thresh) > >> bg_thresh = thresh / 2; > >> > >> But actually we can set vm background dirtiness bigger than > >> vm dirtiness successfully. This behavior may mislead us. > >> We'd better do this validity check at the beginning. > > > > This is an admin only interface. You can screw setting this up even > > when you keep consistency between the background and direct limits. In > > general we do not try to be clever for these knobs because we _expect_ > > admins to do sane things. Why is this any different and why do we need > > to add quite some code to handle one particular corner case? > > > > Of course we expect admins to do the sane things, but not all admins > are expert or faimilar with linux kernel source code. > If we have to read the source code to know what is the right thing to > do, I don't think this is a good interface, even for the admin. Well, it is kind of natural to setup background below the direct limit in general so I am not sure what is so surprising here. Moreover setting a non default drity limits already requires some expertise. It is not like an arbitrary value will work just fine... > Anyway, there's no document on that direct limits should not less than > background limits. Then improve the documentation. > > To be honest I am not entirely sure this is worth the code and the > > future maintenance burden. > I'm not sure if this code is a burden for the future maintenance, but > I think that if we don't introduce this code it is a burden to the > admins. anytime we might need to tweak background vs direct limit we would have to change these checks as well and that sounds like a maint. burden to me. -- Michal Hocko SUSE Labs -- 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