From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with ESMTP id 5723C6B02B2 for ; Sat, 31 Jul 2010 11:31:15 -0400 (EDT) Date: Sat, 31 Jul 2010 16:30:31 +0100 From: Russell King - ARM Linux Subject: Re: [PATCH] Tight check of pfn_valid on sparsemem - v4 Message-ID: <20100731153031.GE27064@n2100.arm.linux.org.uk> References: <20100729161856.GA16420@barrios-desktop> <20100729170313.GB16420@barrios-desktop> <20100729183320.GH18923@n2100.arm.linux.org.uk> <1280436919.16922.11246.camel@nimitz> <20100729221426.GA28699@n2100.arm.linux.org.uk> <1280450338.16922.11735.camel@nimitz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org To: Christoph Lameter Cc: Dave Hansen , Minchan Kim , KAMEZAWA Hiroyuki , Milton Miller , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Mel Gorman , Johannes Weiner , Kukjin Kim List-ID: On Fri, Jul 30, 2010 at 07:48:00AM -0500, Christoph Lameter wrote: > On Thu, 29 Jul 2010, Dave Hansen wrote: > > > SPARSEMEM_EXTREME would be a bit different. It's a 2-level lookup. > > You'd have 16 "section roots", each representing 256MB of address space. > > Each time we put memory under one of those roots, we'd fill in a > > 512-section second-level table, which is designed to always fit into one > > page. If you start at 256MB, you won't waste all those entries. > > That is certain a solution to the !MMU case and it would work very much > like a page table. If you have an MMU then the vmemmap sparsemem > configuration can take advantage of of that to avoid the 2 level lookup. Looking at vmemmap sparsemem, we need to fix it as the page table allocation in there bypasses the arch defined page table setup. This causes a problem if you have 256-entry L2 page tables with no room for the additional Linux VM PTE support bits (such as young, dirty, etc), and need to glue two 256-entry L2 hardware page tables plus a Linux version to store its accounting in each page. See arch/arm/include/asm/pgalloc.h. So this causes a problem with vmemmap: pte_t entry; void *p = vmemmap_alloc_block_buf(PAGE_SIZE, node); if (!p) return NULL; entry = pfn_pte(__pa(p) >> PAGE_SHIFT, PAGE_KERNEL); Are you willing for this stuff to be replaced by architectures as necessary? -- 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