From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753616AbdHXQDW (ORCPT ); Thu, 24 Aug 2017 12:03:22 -0400 Received: from atrey.karlin.mff.cuni.cz ([195.113.26.193]:33221 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753372AbdHXQDT (ORCPT ); Thu, 24 Aug 2017 12:03:19 -0400 Date: Wed, 23 Aug 2017 19:57:09 +0200 From: Pavel Machek To: Michal Hocko Cc: linux-mm@kvack.org, Mel Gorman , Matthew Wilcox , Vlastimil Babka , Neil Brown , "Theodore Ts'o" , Andrew Morton , LKML , Michal Hocko Subject: Re: [RFC PATCH] treewide: remove GFP_TEMPORARY allocation flag Message-ID: <20170823175709.GA22743@xo-6d-61-c0.localdomain> References: <20170728091904.14627-1-mhocko@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170728091904.14627-1-mhocko@kernel.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi! > 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. Dunno. < 1msec probably is temporary, 1 hour probably is not. If it causes problems, can you just #define GFP_TEMPORARY GFP_KERNEL ? Treewide replace, and then starting again goes not look attractive to me. Pavel From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f200.google.com (mail-wr0-f200.google.com [209.85.128.200]) by kanga.kvack.org (Postfix) with ESMTP id 19193440846 for ; Thu, 24 Aug 2017 12:03:24 -0400 (EDT) Received: by mail-wr0-f200.google.com with SMTP id n9so1526136wra.8 for ; Thu, 24 Aug 2017 09:03:24 -0700 (PDT) Received: from atrey.karlin.mff.cuni.cz (atrey.karlin.mff.cuni.cz. [195.113.26.193]) by mx.google.com with ESMTPS id p123si3461054wmp.21.2017.08.24.09.03.18 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 24 Aug 2017 09:03:18 -0700 (PDT) Date: Wed, 23 Aug 2017 19:57:09 +0200 From: Pavel Machek Subject: Re: [RFC PATCH] treewide: remove GFP_TEMPORARY allocation flag Message-ID: <20170823175709.GA22743@xo-6d-61-c0.localdomain> References: <20170728091904.14627-1-mhocko@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170728091904.14627-1-mhocko@kernel.org> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: linux-mm@kvack.org, Mel Gorman , Matthew Wilcox , Vlastimil Babka , Neil Brown , Theodore Ts'o , Andrew Morton , LKML , Michal Hocko Hi! > 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. Dunno. < 1msec probably is temporary, 1 hour probably is not. If it causes problems, can you just #define GFP_TEMPORARY GFP_KERNEL ? Treewide replace, and then starting again goes not look attractive to me. Pavel -- 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