Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751249AbXJVUkI (ORCPT ); Mon, 22 Oct 2007 16:40:08 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750800AbXJVUjz (ORCPT ); Mon, 22 Oct 2007 16:39:55 -0400 Received: from waste.org ([66.93.16.53]:52470 "EHLO waste.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750798AbXJVUjx (ORCPT ); Mon, 22 Oct 2007 16:39:53 -0400 Date: Mon, 22 Oct 2007 15:38:48 -0500 From: Matt Mackall To: Alan Cox Cc: Linus Torvalds , Geert Uytterhoeven , Jens Axboe , Linux Kernel Development , mingo@elte.hu, Linux/m68k Subject: Re: [PATCH 09/10] Change table chaining layout Message-ID: <20071022203848.GI17536@waste.org> References: <1193076664-13652-1-git-send-email-jens.axboe@oracle.com> <1193076664-13652-10-git-send-email-jens.axboe@oracle.com> <20071022211617.31f5c63d@the-village.bc.nu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20071022211617.31f5c63d@the-village.bc.nu> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1685 Lines: 41 On Mon, Oct 22, 2007 at 09:16:17PM +0100, Alan Cox wrote: > On Mon, 22 Oct 2007 12:49:40 -0700 (PDT) > Linus Torvalds wrote: > > > > > > > On Mon, 22 Oct 2007, Geert Uytterhoeven wrote: > > > > > > Better safe than sorry... > > > > > > Is it possible that a chain entry pointer has bit 1 set on architectures > > > (e.g. m68k) where the natural alignment of 32-bit quantities is _2_ bytes, > > > not 4? > > > > Better make sure that such alignment never happens... But no, I don't > > think it will, since these things would generally always have to be > > allocated with an allocator, and the *allocator* won't return 2-byte > > aligned data structures. > > No - but a structure which has other objects in it before the object > being written out may well be 2 byte aligned on M68K and some of the > other externally 16bit platforms - ditto local dynamic objects. Also, the current version of SLOB will return objects aligned at 2 bytes if the architecture allows it. > Why can't we just make the list one item longer than the entry count and > stick a NULL on the end of it like normal people ? Then you need one bit > which ought to be safe for everyone (and if the bit is a macro any CPU > warped enough to have byte alignment is surely going to have top bits > spare...) I'm guessing the extra entry makes slab-like allocators unhappy. -- Mathematics is the supreme nostalgia of our time. - 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/