From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id BEA36C433EF for ; Thu, 7 Apr 2022 09:35:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243833AbiDGJhN (ORCPT ); Thu, 7 Apr 2022 05:37:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53256 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233342AbiDGJhL (ORCPT ); Thu, 7 Apr 2022 05:37:11 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 57CAF7939D for ; Thu, 7 Apr 2022 02:35:11 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id EA1C961AA9 for ; Thu, 7 Apr 2022 09:35:10 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 56B8CC385A4; Thu, 7 Apr 2022 09:35:08 +0000 (UTC) Date: Thu, 7 Apr 2022 10:35:04 +0100 From: Catalin Marinas To: Hyeonggon Yoo <42.hyeyoo@gmail.com> Cc: Will Deacon , Marc Zyngier , Arnd Bergmann , Greg Kroah-Hartman , Andrew Morton , Linus Torvalds , linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 08/10] mm/slab: Allow dynamic kmalloc() minimum alignment Message-ID: References: <20220405135758.774016-1-catalin.marinas@arm.com> <20220405135758.774016-9-catalin.marinas@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 07, 2022 at 06:18:16PM +0900, Hyeonggon Yoo wrote: > On Thu, Apr 07, 2022 at 09:50:23AM +0100, Catalin Marinas wrote: > > On Thu, Apr 07, 2022 at 03:46:37AM +0000, Hyeonggon Yoo wrote: > > > On Tue, Apr 05, 2022 at 02:57:56PM +0100, Catalin Marinas wrote: > > > > --- a/mm/slab_common.c > > > > +++ b/mm/slab_common.c > > > > @@ -838,9 +838,18 @@ void __init setup_kmalloc_cache_index_table(void) > > > > } > > > > } > > > > > > > > -static void __init > > > > +unsigned int __weak arch_kmalloc_minalign(void) > > > > +{ > > > > + return ARCH_KMALLOC_MINALIGN; > > > > +} > > > > + > > > > > > As ARCH_KMALLOC_ALIGN and arch_kmalloc_minalign() may not be same after > > > patch 10, I think s/ARCH_KMALLOC_ALIGN/arch_kmalloc_minalign/g > > > for every user of it would be more correct? > > > > Not if the code currently using ARCH_KMALLOC_MINALIGN needs a constant. > > Yes, there probably are a few places where the code can cope with a > > dynamic arch_kmalloc_minalign() but there are two other cases where a > > constant is needed: > > > > 1. As a BUILD_BUG check because the code is storing some flags in the > > bottom bits of a pointer. A smaller ARCH_KMALLOC_MINALIGN works just > > fine here. > > > > 2. As a static alignment for DMA requirements. That's where the newly > > exposed ARCH_DMA_MINALIGN should be used. > > > > Note that this series doesn't make the situation any worse than before > > since ARCH_DMA_MINALIGN stays at 128 bytes for arm64. Current users can > > evolve to use a dynamic alignment in future patches. My main aim with > > this series is to be able to create kmalloc-64 caches on arm64. > > AFAIK there are bunch of drivers that directly calls kmalloc(). Well, lots of drivers call kmalloc() ;). > It becomes tricky when e.g.) a driver allocates just 32 bytes, > but architecture requires it to be 128-byte aligned. That's the current behaviour, a 32 byte allocation would return an object from kmalloc-128. I want to reduce this to at least kmalloc-64 (or smaller) if the CPU/SoC allows it. > That's why everything allocated from kmalloc() need to be aligned in > ARCH_DMA_MINALIGN. I don't get your conclusion here. Would you mind explaining? > So I'm yet skeptical on decoupling ARCH_DMA/KMALLOC_MINALIGN. Instead > of decoupling it, I'm more into dynamically decreasing it. The reason for decoupling is mostly that there are some static uses of ARCH_KMALLOC_MINALIGN as per point 1 above. The other is the __assume_kmalloc_alignment attribute. We shouldn't have such assumed alignment larger than what a dynamic kmalloc() would return. To me it makes a lot more sense for ARCH_KMALLOC_MINALIGN to be the minimum guaranteed in a kernel build but kmalloc() returning a larger alignment at run-time than the other way around. Thanks. -- Catalin