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 X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9DCE2ECE560 for ; Mon, 24 Sep 2018 17:49:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5BDD920C0A for ; Mon, 24 Sep 2018 17:49:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=kernel-dk.20150623.gappssmtp.com header.i=@kernel-dk.20150623.gappssmtp.com header.b="fJ2E6rxd" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5BDD920C0A Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.dk Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729883AbeIXXw6 (ORCPT ); Mon, 24 Sep 2018 19:52:58 -0400 Received: from mail-it1-f194.google.com ([209.85.166.194]:55496 "EHLO mail-it1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728480AbeIXXw5 (ORCPT ); Mon, 24 Sep 2018 19:52:57 -0400 Received: by mail-it1-f194.google.com with SMTP id c23-v6so3597832itd.5 for ; Mon, 24 Sep 2018 10:49:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20150623.gappssmtp.com; s=20150623; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=gvjrS2GhMjxr+neoEq5hTDN2Nb/weh/WOvMr8kIrZbM=; b=fJ2E6rxdg/NEPdj62G0AXDI+K9c7Mnwv5Hyqi6/PAdZvqs3HuqMESAuvEARKfN03wh Mfw65Nv4b31mm+Zt7tuXNsNT48CmSwuGt7QEmIMjVWH9p0hnEvv23YxOmdV9/cdyNi+u UEaFUlX7kfQw8YE6q2OlIspQi08T9lpMlUUR/iHdDG7AlOQRIEnpovxmRwaD6NZIm/ic Kjzq3XVM51/KXa8F+tbRuZYWoNEYZyNlP/CSr+61saS+RqOIYPbwqfmlCpQJ3ajp1VcV TFjTWtE+mv9GPvh9UM+4Ng7F15OKDbxSOZu+OCtGVwLQoPmN0xrTr4aZxkBNaNOjK5QC dIwg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=gvjrS2GhMjxr+neoEq5hTDN2Nb/weh/WOvMr8kIrZbM=; b=N8CiAw26W67+g9tIZQ1iU0xkWRpHkE4wedEKujZmnKMOpqfgp/mgf/bxZHHffjDQVO rBqlCm+b7LIXzq69EHnCKpiggGyUXD4GB6TWyuqZSc62NKJLvED6NwGCC533UxX/Fil6 P7d8DBewMPgxx6elHPq28B1gU77NfOOXY+3l7f2QDBDiickm0VIHqJ26V0xQ2Wa1hRWW kCrS+aDP8XKjGfm8E6ZV7MeMSD0subUEbtSG2gzy9NFjga2KYgd7HgdPx0sGI9XYORGi K/bfROwWLndazGfDxl1F3Pcgu0jvb9CPL4rZFxVnNbsSpcuhlfhvjYQ0ShposBPXMC9r j3RA== X-Gm-Message-State: APzg51CLk34Mwgnl7EU+vhRLr8k612hfzuflMKp9v9SU4wqYCKigWEkZ Mjn+bHTWD9qKNs6h9niuuy+VJw== X-Google-Smtp-Source: ANB0VdarV49goD/cgmdKgMDyRnKL21Wbq/nJlpZiltaCHmWzk+yFOr/eYEqe+4lHTcS4DrYIxm2zCg== X-Received: by 2002:a02:94e5:: with SMTP id x92-v6mr10343077jah.67.1537811379173; Mon, 24 Sep 2018 10:49:39 -0700 (PDT) Received: from [192.168.1.56] ([216.160.245.98]) by smtp.gmail.com with ESMTPSA id v6-v6sm2298769ioh.42.2018.09.24.10.49.37 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 24 Sep 2018 10:49:37 -0700 (PDT) Subject: Re: block: DMA alignment of IO buffer allocated from slab To: Christopher Lameter , Christoph Hellwig Cc: Vitaly Kuznetsov , Ming Lei , linux-block , linux-mm , Linux FS Devel , "open list:XFS FILESYSTEM" , Dave Chinner , Linux Kernel Mailing List , Ming Lei References: <20180920063129.GB12913@lst.de> <87h8ij0zot.fsf@vitty.brq.redhat.com> <20180921130504.GA22551@lst.de> <010001660c54fb65-b9d3a770-6678-40d0-8088-4db20af32280-000000@email.amazonses.com> From: Jens Axboe Message-ID: <1f88f59a-2cac-e899-4c2e-402e919b1034@kernel.dk> Date: Mon, 24 Sep 2018 11:49:36 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0 MIME-Version: 1.0 In-Reply-To: <010001660c54fb65-b9d3a770-6678-40d0-8088-4db20af32280-000000@email.amazonses.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/24/18 10:06 AM, Christopher Lameter wrote: > On Fri, 21 Sep 2018, Christoph Hellwig wrote: > >> On Fri, Sep 21, 2018 at 03:04:18PM +0200, Vitaly Kuznetsov wrote: >>> Christoph Hellwig writes: >>> >>>> On Wed, Sep 19, 2018 at 05:15:43PM +0800, Ming Lei wrote: >>>>> 1) does kmalloc-N slab guarantee to return N-byte aligned buffer? If >>>>> yes, is it a stable rule? >>>> >>>> This is the assumption in a lot of the kernel, so I think if somethings >>>> breaks this we are in a lot of pain. >>> >>> It seems that SLUB debug breaks this assumption. Kernel built with >>> >>> CONFIG_SLUB_DEBUG=y >>> CONFIG_SLUB=y >>> CONFIG_SLUB_DEBUG_ON=y >> >> Looks like we should fix SLUB debug then.. > > Nope. We need to not make unwarranted assumptions. Alignment is guaranteed > to ARCH_KMALLOC_MINALIGN for kmalloc requests. Fantasizing about > alighments and guessing from alignments that result on a particular > hardware and slab configuration that these are general does not work. The summary is that, no, kmalloc(N) is not N-1 aligned and nobody should rely on that. On the block side, a few drivers set DMA alignment to the sector size. Given that things seem to Just Work, even with XFS doing kmalloc(512) and submitting IO with that, I think we can fairly safely assume that most of those drivers are just being overly cautious and are probably quite fine with 4/8 byte alignment. The situation is making me a little uncomfortable, though. If we export such a setting, we really should be honoring it... -- Jens Axboe