From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753544AbcA0Fwr (ORCPT ); Wed, 27 Jan 2016 00:52:47 -0500 Received: from LGEAMRELO11.lge.com ([156.147.23.51]:57025 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752189AbcA0Fwm (ORCPT ); Wed, 27 Jan 2016 00:52:42 -0500 X-Original-SENDERIP: 156.147.1.121 X-Original-MAILFROM: iamjoonsoo.kim@lge.com X-Original-SENDERIP: 10.177.222.138 X-Original-MAILFROM: iamjoonsoo.kim@lge.com Date: Wed, 27 Jan 2016 14:52:43 +0900 From: Joonsoo Kim To: Dan Williams Cc: Rik van Riel , linux-nvdimm , Dave Hansen , "linux-kernel@vger.kernel.org" , Sudip Mukherjee , Linux MM , Ingo Molnar , Mel Gorman , "H. Peter Anvin" , Jerome Glisse , Andrew Morton , Christoph Hellwig Subject: Re: [RFC PATCH] mm: support CONFIG_ZONE_DEVICE + CONFIG_ZONE_DMA Message-ID: <20160127055243.GA8568@js1304-P5Q-DELUXE> References: <20160126141152.e1043d14502dcca17813afb3@linux-foundation.org> <20160126145153.44e4f38b04200209d133c0a3@linux-foundation.org> <20160127011817.GA7398@js1304-P5Q-DELUXE> <20160127021515.GA7562@js1304-P5Q-DELUXE> <20160127035215.GA7813@js1304-P5Q-DELUXE> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 On Tue, Jan 26, 2016 at 08:26:24PM -0800, Dan Williams wrote: > On Tue, Jan 26, 2016 at 7:52 PM, Joonsoo Kim wrote: > > On Tue, Jan 26, 2016 at 07:23:59PM -0800, Dan Williams wrote: > >> On Tue, Jan 26, 2016 at 6:15 PM, Joonsoo Kim wrote: > >> > On Tue, Jan 26, 2016 at 05:37:38PM -0800, Dan Williams wrote: > >> >> On Tue, Jan 26, 2016 at 5:18 PM, Joonsoo Kim wrote: > >> [..] > >> >> > Please refer my previous attempt to add a new zone, ZONE_CMA. > >> >> > > >> >> > https://lkml.org/lkml/2015/2/12/84 > >> >> > > >> >> > It salvages a bit from SECTION_WIDTH by increasing section size. > >> >> > Similarly, I guess we can reduce NODE_WIDTH if needed although > >> >> > it could cause to reduce maximum node size. > >> >> > >> >> Dave pointed out to me that LAST__PID_SHIFT might be a better > >> >> candidate to reduce to 7 bits. That field is for storing pids which > >> >> are already bigger than 8 bits. If it is relying on the fact that > >> >> pids don't rollover very often then likely the impact of 7-bits > >> >> instead of 8 will be minimal. > >> > > >> > Hmm... I'm not sure it's possible or not, but, it looks not a general > >> > solution. It will solve your problem because you are using 64 bit arch > >> > but other 32 bit archs can't get the benefit. > >> > >> This is where the ZONE_CMA and ZONE_DEVICE efforts diverge. > >> ZONE_DEVICE is meant to enable DMA access to hundreds of gigagbytes of > >> persistent memory. A 64-bit-only limitation for ZONE_DEVICE is > >> reasonable. > > > > Yes, but, my point is that if someone need another zone like as > > ZONE_CMA, they couldn't get the benefit from this change. They need to > > re-investigate what bits they can reduce and need to re-do all things. > > > > If it is implemented more generally at this time, it can relieve their > > burden and less churn the code. It would be helpful for maintainability. > > I agree in principle that finding a 32-bit compatible solution is > desirable, but it simply may not be feasible. Okay. > > For now, I'll help with auditing the existing bits so we can enumerate > the tradeoffs. Thanks! :) > Hmm, one tradeoff that comes to mind for 32-bit is sacrificing > ZONE_HIGHMEM, for ZONE_CMA. Are there configurations that need both > enabled? If a platform needs highmem it really should be using a > 64-bit kernel (if possible), desire for ZONE_CMA might be a nice > encouragement to lessen the prevalence of highmem. I guess that it's not possible. There are many systems that need both. I don't think deeply, but, there is another option for ZONE_CMA. It can share ZONE_MOVABLE because their chracteristic is roughly same in view of MM. I will think more. Thanks.