From: Manfred Spraul Subject: Re: [PATCH 1/4] mm: Move ARCH_SLAB_MINALIGN and ARCH_KMALLOC_MINALIGN to Date: Wed, 19 May 2010 20:03:00 +0200 Message-ID: <4BF427D4.40600@colorfullife.com> References: <1274266725.6930.9823.camel@macbook.infradead.org> <1274266902.6930.9834.camel@macbook.infradead.org> <20100519133028.GA26659@sig21.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: David Woodhouse , Herbert Xu , David Miller , penberg@cs.helsinki.fi, mpm@selenic.com, ken@codelabs.ch, geert@linux-m68k.org, michael-dev@fami-braun.de, linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, anemo@mba.ocn.ne.jp To: Johannes Stezenbach Return-path: Received: from mail-fx0-f46.google.com ([209.85.161.46]:61550 "EHLO mail-fx0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753479Ab0ESSB0 (ORCPT ); Wed, 19 May 2010 14:01:26 -0400 In-Reply-To: <20100519133028.GA26659@sig21.net> Sender: linux-crypto-owner@vger.kernel.org List-ID: On 05/19/2010 03:30 PM, Johannes Stezenbach wrote: > Hi, > > I have some comments/questions, I hope it's not too silly: > > On Wed, May 19, 2010 at 12:01:42PM +0100, David Woodhouse wrote: > >> +#ifndef ARCH_KMALLOC_MINALIGN >> +/* >> + * Enforce a minimum alignment for the kmalloc caches. >> + * Usually, the kmalloc caches are cache_line_size() aligned, except when >> + * DEBUG and FORCED_DEBUG are enabled, then they are BYTES_PER_WORD aligned. >> + * Some archs want to perform DMA into kmalloc caches and need a guaranteed >> + * alignment larger than the alignment of a 64-bit integer. >> + * ARCH_KMALLOC_MINALIGN allows that. >> + * Note that increasing this value may disable some debug features. >> + */ >> +#define ARCH_KMALLOC_MINALIGN __alignof__(unsigned long long) >> +#endif >> > I think the comment is confusing. IIRC kmalloc() API guarantees that > the allocated buffer is suitable for DMA, so if cache coherence is not > handled by hardware the arch might need to set this to the cache line size, > and that's what ARCH_KMALLOC_MINALIGN is about. Nothing else. > Is this text better? /* * Enforce a minimum alignment for the kmalloc caches. * kmalloc allocations are guaranteed to be BYTES_PER_WORD aligned - sizeof(void*) * If an arch needs a larger guarantee (e.g. cache_line_size due to DMA), then it * must use ARCH_KMALLOC_MINALIGN to enforce that. * Note: Do not set ARCH_KMALLOC_MINALIGN for performance reasons. * Unless debug options are enabled, the kernel uses cache_line_size() automatically. */ >> +#ifndef ARCH_SLAB_MINALIGN >> +/* >> + * Enforce a minimum alignment for all caches. >> + * Intended for archs that get misalignment faults even for BYTES_PER_WORD >> + * aligned buffers. Includes ARCH_KMALLOC_MINALIGN. >> + * If possible: Do not enable this flag for CONFIG_DEBUG_SLAB, it disables >> + * some debug features. >> + */ >> +#define ARCH_SLAB_MINALIGN 0 >> +#endif >> > Why is this needed at all? If code calls kmem_cache_create() > with wrong align parameter, or has wrong expectations wrt kmalloc() > alignment guarantees, this code needs to be fixed? > I mean, portable code cannot assume that unaligned accesses work? > ARM uses 8 bytes. I don't know why. A 32-bit arch and unaligned 64-bit loads are not supported? -- Manfred