From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751993AbdG1NPH (ORCPT ); Fri, 28 Jul 2017 09:15:07 -0400 Received: from mx2.suse.de ([195.135.220.15]:39022 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751071AbdG1NPG (ORCPT ); Fri, 28 Jul 2017 09:15:06 -0400 Subject: Re: [RFC PATCH] treewide: remove GFP_TEMPORARY allocation flag To: Michal Hocko , linux-mm@kvack.org Cc: Mel Gorman , Matthew Wilcox , Neil Brown , "Theodore Ts'o" , Andrew Morton , LKML , Michal Hocko References: <20170728091904.14627-1-mhocko@kernel.org> From: Vlastimil Babka Message-ID: <7ba2635d-68bd-ee1a-caa2-3ff571c7a3ee@suse.cz> Date: Fri, 28 Jul 2017 15:15:03 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170728091904.14627-1-mhocko@kernel.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/28/2017 11:19 AM, Michal Hocko wrote: > From: Michal Hocko > > GFP_TEMPORARY has been introduced by e12ba74d8ff3 ("Group short-lived > and reclaimable kernel allocations") along with __GFP_RECLAIMABLE. It's > primary motivation was to allow users to tell that an allocation is > short lived and so the allocator can try to place such allocations close > together and prevent long term fragmentation. As much as this sounds > like a reasonable semantic it becomes much less clear when to use the > highlevel GFP_TEMPORARY allocation flag. How long is temporary? Can > the context holding that memory sleep? Can it take locks? It seems > there is no good answer for those questions. > > The current implementation of GFP_TEMPORARY is basically > GFP_KERNEL | __GFP_RECLAIMABLE which in itself is tricky because > basically none of the existing caller provide a way to reclaim the > allocated memory. So this is rather misleading and hard to evaluate for > any benefits. > > I have checked some random users and none of them has added the flag > with a specific justification. I suspect most of them just copied from > other existing users and others just thought it might be a good idea > to use without any measuring. This suggests that GFP_TEMPORARY just > motivates for cargo cult usage without any reasoning. > > I believe that our gfp flags are quite complex already and especially > those with highlevel semantic should be clearly defined to prevent from > confusion and abuse. Therefore I propose dropping GFP_TEMPORARY and > replace all existing users to simply use GFP_KERNEL. Please note that > SLAB users with shrinkers will still get __GFP_RECLAIMABLE heuristic > and so they will be placed properly for memory fragmentation prevention. > > I can see reasons we might want some gfp flag to reflect shorterm > allocations but I propose starting from a clear semantic definition and > only then add users with proper justification. > > Signed-off-by: Michal Hocko Yes, it's best we remove it. Acked-by: Vlastimil Babka