From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752704Ab0JNAYT (ORCPT ); Wed, 13 Oct 2010 20:24:19 -0400 Received: from terminus.zytor.com ([198.137.202.10]:50794 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752063Ab0JNAYS (ORCPT ); Wed, 13 Oct 2010 20:24:18 -0400 Message-ID: <4CB64D8F.9080800@zytor.com> Date: Wed, 13 Oct 2010 17:23:43 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.9) Gecko/20100921 Fedora/3.1.4-1.fc13 Thunderbird/3.1.4 MIME-Version: 1.0 To: David Rientjes CC: Ingo Molnar , Thomas Gleixner , Casey Dahlin , x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [patch] x86: allow ZONE_DMA to be configurable References: In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/13/2010 05:15 PM, David Rientjes wrote: > ZONE_DMA is unnecessary for a large number of machines that do not > require addressing in the lower 16MB of memory because they do not use > ISA devices with 16-bit address registers (plus one page byte register). > > This patch allows users to disable ZONE_DMA for x86 if they know they > will not be using such devices with their kernel. > > This prevents the VM from unnecessarily reserving a ratio of memory > (defaulting to 1/256th of system capacity) with lowmem_reserve_ratio > for such allocations when it will never be used. This isn't true. There are many, many devices other than ISA devices which need access to a restricted-memory pool because of hardware DMA limitations. This seems like a really bad idea. A much better idea would be to have a runtime option of setting the reservation ratio, if recovering no more than 1/66th of a gigabyte matters so much to you. -hpa