Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756213Ab0DFUN5 (ORCPT ); Tue, 6 Apr 2010 16:13:57 -0400 Received: from terminus.zytor.com ([198.137.202.10]:59940 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753224Ab0DFUNw (ORCPT ); Tue, 6 Apr 2010 16:13:52 -0400 Message-ID: <4BBB93C2.1070108@zytor.com> Date: Tue, 06 Apr 2010 13:04:18 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4 MIME-Version: 1.0 To: Joel Fernandes CC: Frank Hu , hayfeng Lee , linux-kernel@vger.kernel.org, linux-kernel@zh-kernel.org, kernelnewbies@nl.linux.org Subject: Re: why choose 896MB to the start point of ZONE_HIGHMEM References: <4BBB7AC9.5060008@zytor.com> <4BBB8F07.60401@zytor.com> 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 Content-Length: 1609 Lines: 39 On 04/06/2010 01:01 PM, Joel Fernandes wrote: > Hi Peter, > > On Wed, Apr 7, 2010 at 1:14 AM, H. Peter Anvin wrote: >> On 04/06/2010 12:20 PM, Frank Hu wrote: >>>> >>>> The ELF ABI specifies that user space has 3 GB available to it. That >>>> leaves 1 GB for the kernel. The kernel, by default, uses 128 MB for I/O >>>> mapping, vmalloc, and kmap support, which leaves 896 MB for LOWMEM. >>>> >>>> All of these boundaries are configurable; with PAE enabled the user >>>> space boundary has to be on a 1 GB boundary. >>>> >>> >>> the VM split is also configurable when building the kernel (for 32-bit >>> processors). >> >> I did say "all these boundaries are configurable". Rather explicitly. >> > > I thought the 896 MB was a hardware limitation on 32 bit architectures > and something that cannot be configured? Or am I missing something > here? Also the vm-splits refer to "virtual memory" . While ZONE_* and > the 896MB we were discussing refers to "physical memory". How then is > discussing about vm splits pertinent here? > It's not a hardware limitation. Rather, it has to do with how the 4 GB of virtual address space is carved up. LOWMEM specifically refers to the amount of memory which is permanently mapped into the virtual address space, whereas HIGHMEM is mapped in and out on demand -- a fairly expensive operation. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/