From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752765AbYIJR0E (ORCPT ); Wed, 10 Sep 2008 13:26:04 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751446AbYIJRZx (ORCPT ); Wed, 10 Sep 2008 13:25:53 -0400 Received: from outbound-wa4.frontbridge.com ([216.32.181.16]:43133 "EHLO WA4EHSOBE003.bigfish.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751378AbYIJRZw (ORCPT ); Wed, 10 Sep 2008 13:25:52 -0400 X-BigFish: VPS-24(zz1432R98dR1805M936fQ873fnzz10d3izzz32i6bh43j61h) X-Spam-TCS-SCL: 0:0 X-FB-SS: 5, X-WSS-ID: 0K6ZPQG-04-NMP-01 Date: Wed, 10 Sep 2008 19:25:33 +0200 From: Joerg Roedel To: FUJITA Tomonori CC: linux-kernel@vger.kernel.org, mingo@elte.hu Subject: Re: [PATCH] x86: avoid unnecessary low zone allocation in AMD IOMMU's alloc_coherent Message-ID: <20080910172533.GB32286@amd.com> References: <20080910152918.GD24392@amd.com> <20080911012932D.fujita.tomonori@lab.ntt.co.jp> <20080910170504.GA32286@amd.com> <20080911021457K.fujita.tomonori@lab.ntt.co.jp> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20080911021457K.fujita.tomonori@lab.ntt.co.jp> User-Agent: mutt-ng/devel-r804 (Linux) X-OriginalArrivalTime: 10 Sep 2008 17:25:33.0104 (UTC) FILETIME=[3AD18300:01C9136A] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 11, 2008 at 02:15:22AM +0900, FUJITA Tomonori wrote: > On Wed, 10 Sep 2008 19:05:04 +0200 > Joerg Roedel wrote: > > > On Thu, Sep 11, 2008 at 01:29:54AM +0900, FUJITA Tomonori wrote: > > > On Wed, 10 Sep 2008 17:29:18 +0200 > > > Joerg Roedel wrote: > > > > > > > What hacks do you need with __get_free_pages? The memory it returns is > > > > _always_ aligned at its size. > > > > > > Is it guaranteed (documented somewhere) ? > > > > I don't know if there is a formal definition for it. It is documented in > > some books about the Linux kernel (I read this in some book the first > > time). This alignment results from the buddy algorithm the page alloctor > > uses. You can definitly rely on that. > > I meant, if it's not documented as a guaranteed feature (not just the > characteristic of the current code), it could change any time. It will never change. I am sure about this. A lot of code relys on this like the HugeTLBfs implementations for example. In fact, it may be possible that the requirement to the dma_alloc_coherent function derive from this behavior of the Linux page allocator (to not break anything when the dma api was introduced). Joerg -- | AMD Saxony Limited Liability Company & Co. KG Operating | Wilschdorfer Landstr. 101, 01109 Dresden, Germany System | Register Court Dresden: HRA 4896 Research | General Partner authorized to represent: Center | AMD Saxony LLC (Wilmington, Delaware, US) | General Manager of AMD Saxony LLC: Dr. Hans-R. Deppe, Thomas McCoy