From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id 820B16B0003 for ; Fri, 22 Jun 2018 05:09:39 -0400 (EDT) Received: by mail-ed1-f72.google.com with SMTP id i19-v6so343788eds.20 for ; Fri, 22 Jun 2018 02:09:39 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id g20-v6si3101192edq.282.2018.06.22.02.09.38 for (version=TLS1 cipher=AES128-SHA bits=128/128); Fri, 22 Jun 2018 02:09:38 -0700 (PDT) Date: Fri, 22 Jun 2018 11:09:35 +0200 From: Michal Hocko Subject: Re: dm bufio: Reduce dm_bufio_lock contention Message-ID: <20180622090935.GT10465@dhcp22.suse.cz> References: <20180615073201.GB24039@dhcp22.suse.cz> <20180615115547.GH24039@dhcp22.suse.cz> <20180615130925.GI24039@dhcp22.suse.cz> <20180619104312.GD13685@dhcp22.suse.cz> <20180622090151.GS10465@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180622090151.GS10465@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Mikulas Patocka Cc: jing xia , Mike Snitzer , agk@redhat.com, dm-devel@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org On Fri 22-06-18 11:01:51, Michal Hocko wrote: > On Thu 21-06-18 21:17:24, Mikulas Patocka wrote: [...] > > What about this patch? If __GFP_NORETRY and __GFP_FS is not set (i.e. the > > request comes from a block device driver or a filesystem), we should not > > sleep. > > Why? How are you going to audit all the callers that the behavior makes > sense and moreover how are you going to ensure that future usage will > still make sense. The more subtle side effects gfp flags have the harder > they are to maintain. So just as an excercise. Try to explain the above semantic to users. We currently have the following. * __GFP_NORETRY: The VM implementation will try only very lightweight * memory direct reclaim to get some memory under memory pressure (thus * it can sleep). It will avoid disruptive actions like OOM killer. The * caller must handle the failure which is quite likely to happen under * heavy memory pressure. The flag is suitable when failure can easily be * handled at small cost, such as reduced throughput * __GFP_FS can call down to the low-level FS. Clearing the flag avoids the * allocator recursing into the filesystem which might already be holding * locks. So how are you going to explain gfp & (__GFP_NORETRY | ~__GFP_FS)? What is the actual semantic without explaining the whole reclaim or force users to look into the code to understand that? What about GFP_NOIO | __GFP_NORETRY? What does it mean to that "should not sleep". Do all shrinkers have to follow that as well? -- Michal Hocko SUSE Labs