From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=MAILING_LIST_MULTI,SPF_PASS, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 09FE1C43143 for ; Fri, 22 Jun 2018 09:09:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B8E1523F69 for ; Fri, 22 Jun 2018 09:09:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B8E1523F69 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751312AbeFVJJi (ORCPT ); Fri, 22 Jun 2018 05:09:38 -0400 Received: from mx2.suse.de ([195.135.220.15]:55002 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751086AbeFVJJh (ORCPT ); Fri, 22 Jun 2018 05:09:37 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (charybdis-ext-too.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 88059AC17; Fri, 22 Jun 2018 09:09:36 +0000 (UTC) Date: Fri, 22 Jun 2018 11:09:35 +0200 From: Michal Hocko To: Mikulas Patocka Cc: jing xia , Mike Snitzer , agk@redhat.com, dm-devel@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org 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> User-Agent: Mutt/1.9.5 (2018-04-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.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