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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 17B52C433F5 for ; Thu, 14 Apr 2022 13:53:09 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 7E71C6B0071; Thu, 14 Apr 2022 09:53:08 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 794BE6B0073; Thu, 14 Apr 2022 09:53:08 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 635C06B0074; Thu, 14 Apr 2022 09:53:08 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay.hostedemail.com [64.99.140.25]) by kanga.kvack.org (Postfix) with ESMTP id 4E3716B0071 for ; Thu, 14 Apr 2022 09:53:08 -0400 (EDT) Received: from smtpin10.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay07.hostedemail.com (Postfix) with ESMTP id 1823C22EBA for ; Thu, 14 Apr 2022 13:53:08 +0000 (UTC) X-FDA: 79355626056.10.3563245 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by imf23.hostedemail.com (Postfix) with ESMTP id 738D8140003 for ; Thu, 14 Apr 2022 13:53:07 +0000 (UTC) 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 DB9C661E30 for ; Thu, 14 Apr 2022 13:53:06 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4C77FC385AB for ; Thu, 14 Apr 2022 13:53:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1649944386; bh=fGiKea3HNEw5JXDV2PTTJNXYtHuJBrMkih6xGRJRshY=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=twNoeCqMgJ0nWqHCGUfbSz+ef1tYvn4jyhWEWnrk9vXIJwo8la4yhcnWAxHNi9jd9 jzW+xApAezra8KYbvz2uDAbbhUy3PsbzS9IbmJJDLd3xcZyJJibki9MIa6WCwDQ900 2UuVy1S+aIsjdfVkKCACSEj0aAuJp8mrrTaHBFx5f3/dAgTcNF8ExhOLSY7I6ntOUs F8bfII7DZU12RV7/7IyX3HMIqApj8ghOnZ7yjhB962qDyjdA1dv0+qWuCQZSwa6fVA Lj46M8RK799c+iBNnzrvSAyFD6XusJ2hEDDBV5VT8bo56wYZ838J36A9s5K2LBjCL4 asTMnt9rgBndA== Received: by mail-oi1-f172.google.com with SMTP id r8so5442892oib.5 for ; Thu, 14 Apr 2022 06:53:06 -0700 (PDT) X-Gm-Message-State: AOAM533unqg5g7uq2tQsITwGLO+WSrHbdd8HvBEKfpB5nlbvabanCDN3 hmFds509pFB24vnrElRjqpojHWiXP6o7H2mJ5N8= X-Google-Smtp-Source: ABdhPJw5QtE/GX85YaVo/7SirG8M1THNaumOcNkM0p8q2w5uHOUPZSpdDXTGiIo2++Em9HgRc3WS+wxVtmIzZrwuASo= X-Received: by 2002:a05:6808:1513:b0:2fa:7a40:c720 with SMTP id u19-20020a056808151300b002fa7a40c720mr1379987oiw.126.1649944385403; Thu, 14 Apr 2022 06:53:05 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Ard Biesheuvel Date: Thu, 14 Apr 2022 15:52:53 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN To: Greg Kroah-Hartman Cc: Linus Torvalds , Catalin Marinas , Herbert Xu , Will Deacon , Marc Zyngier , Arnd Bergmann , Andrew Morton , Linux Memory Management List , Linux ARM , Linux Kernel Mailing List , "David S. Miller" Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 738D8140003 X-Stat-Signature: fitz91kgjkc4hkk6qcauqpq75d3pthwa Authentication-Results: imf23.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=twNoeCqM; dmarc=pass (policy=none) header.from=kernel.org; spf=pass (imf23.hostedemail.com: domain of ardb@kernel.org designates 139.178.84.217 as permitted sender) smtp.mailfrom=ardb@kernel.org X-Rspam-User: X-HE-Tag: 1649944387-614652 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, 14 Apr 2022 at 07:38, Greg Kroah-Hartman wrote: > > On Wed, Apr 13, 2022 at 09:53:24AM -1000, Linus Torvalds wrote: > > On Tue, Apr 12, 2022 at 10:47 PM Catalin Marinas > > wrote: > > > > > > I agree. There is also an implicit expectation that the DMA API works on > > > kmalloc'ed buffers and that's what ARCH_DMA_MINALIGN is for (and the > > > dynamic arch_kmalloc_minalign() in this series). But the key point is > > > that the driver doesn't need to know the CPU cache topology, coherency, > > > the DMA API and kmalloc() take care of these. > > > > Honestly, I think it would probably be worth discussing the "kmalloc > > DMA alignment" issues. > > > > 99.9% of kmalloc users don't want to do DMA. > > > > And there's actually a fair amount of small kmalloc for random stuff. > > Right now on my laptop, I have > > > > kmalloc-8 16907 18432 8 512 1 : ... > > > > according to slabinfo, so almost 17 _thousand_ allocations of 8 bytes. > > > > It's all kinds of sad if those allocations need to be 64 bytes in size > > just because of some silly DMA alignment issue, when none of them want > > it. > > Actually, the alignment for non-cache coherent DMA is 128 bytes on arm64, not 64 bytes. > > Yeah, yeah, wasting a megabyte of memory is "just a megabyte" these > > days. Which is crazy. It's literally memory that could have been used > > for something much more useful than just pure and utter waste. > > > > I think we could and should just say "people who actually require DMA > > accesses should say so at kmalloc time". We literally have that > > GFP_DMA and ZOME_DMA for various historical reasons, so we've been > > able to do that before. > > > > No, that historical GFP_DMA isn't what arm64 wants - it's the old > > crazy "legacy 16MB DMA" thing that ISA DMA used to have. > > > > But the basic issue was true then, and is true now - DMA allocations > > are fairly special, and should not be that hard to just mark as such. > > "fairly special" == "all USB transactions", so it will take a lot of > auditing here. I think also many SPI controllers require this and maybe > I2C? Perhaps other bus types do as well. > > So please don't make this change without some way of figuring out just > what drivers need to be fixed up, as it's going to be a lot... > Yeah, the current de facto contract of being able to DMA map anything that was allocated via the linear map makes it quite hard to enforce the use of dma_kmalloc() for this. What we might do, given the fact that only inbound non-cache coherent DMA is problematic, is dropping the kmalloc alignment to 8 like on x86, and falling back to bounce buffering when a misaligned, non-cache coherent inbound DMA mapping is created, using the SWIOTLB bounce buffering code that we already have, and is already in use on most affected systems for other reasons (i.e., DMA addressing limits) This will cause some performance regressions, but in a way that seems fixable to me: taking network drivers as an example, the RX buffers that are filled using inbound DMA are typically owned by the driver itself, which could be updated to round up its allocations and DMA mappings. Block devices typically operate on quantities that are aligned sufficiently already. In other cases, we will likely notice if/when this fallback is taken on a hot path, but if we don't, at least we know a bounce buffer is being used whenever we cannot perform the DMA safely in-place.