From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S264685AbUHWOb0 (ORCPT ); Mon, 23 Aug 2004 10:31:26 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S264668AbUHWOb0 (ORCPT ); Mon, 23 Aug 2004 10:31:26 -0400 Received: from ebiederm.dsl.xmission.com ([166.70.28.69]:43946 "EHLO ebiederm.dsl.xmission.com") by vger.kernel.org with ESMTP id S264697AbUHWOas (ORCPT ); Mon, 23 Aug 2004 10:30:48 -0400 To: "Mukund JB." Cc: Subject: Re: Support for HIGHMEM ...can any one explain my Q&A .. References: <4EE0CBA31942E547B99B3D4BFAB348110B1385@mail.esn.co.in> From: ebiederm@xmission.com (Eric W. Biederman) Date: 23 Aug 2004 08:29:28 -0600 In-Reply-To: <4EE0CBA31942E547B99B3D4BFAB348110B1385@mail.esn.co.in> Message-ID: User-Agent: Gnus/5.0808 (Gnus v5.8.8) Emacs/21.2 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org "Mukund JB." writes: > Hi all, > Sorry if I asked any thing that basic level stuff? > This is the paasge I found in the net while surfing for details about > HIGHMEM stuff. > > During 2.3 kernel development (I think), "HIGHMEM" support was added. > Normally, the kernel can only address (4GB-PAGE_OFFSET)/PAGE_SIZE pages > of RAM, since all physical pages must be mapped to kernel addresses > between PAGE_OFFSET and 4GB. (So if PAGE_OFFSET is 3GB, only 1GB of > physical RAM can be used - not even that, in practice, due to fixed > kernel mappings and so forth.) The HIGHMEM patches allow the kernel to > use more than 1G of memory by mapping the additional pages into the high > part of the kernel address space just below 4GB as necessary. They also > allow high-memory pages to be mapped into user process address space. > > > ***) Does the above passage mean that PAGE_OFFSET is the starting > address of my RAM ? No. PAGE_OFFSET is the virtual address at which physical address 0 is mapped into an arch/i386 kernel. > I understood so from the below line > "since all physical pages must be mapped to kernel addresses between > PAGE_OFFSET and 4GB". > > ***) Does it mean that the lowest portion of the 4GB os nothing but RAM > ? No. 0-PAGE_OFFSET is the virtual address space provided to user space. So it has an arbitrary mapping to virtual memory. Eric