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=-8.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 9AE0EC433ED for ; Sun, 11 Apr 2021 22:36:54 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id C6406611AD for ; Sun, 11 Apr 2021 22:36:53 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C6406611AD Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4FJRYv6Qhgz3c0C for ; Mon, 12 Apr 2021 08:36:51 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256 header.s=casper.20170209 header.b=ciV9TA9s; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=none (no SPF record) smtp.mailfrom=infradead.org (client-ip=2001:8b0:10b:1236::1; helo=casper.infradead.org; envelope-from=willy@infradead.org; receiver=) Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256 header.s=casper.20170209 header.b=ciV9TA9s; dkim-atps=neutral Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4FJRYP15Bmz2yRY for ; Mon, 12 Apr 2021 08:36:20 +1000 (AEST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=lvW0/1imLWgxPdvw89GQU7hXjnk5amCWvOf6j0ZGMxs=; b=ciV9TA9sI2hcmMk+5mvn6+GMnQ NVm1V6dO3xEncplJkkDlPA7TgOj6ULsrH2ZS8g/+oRMVmxes2/l/PJUJomi09t748PLbrirAblFzt 9+iYMHD4kchH5/T/s2uhJzpGrDIP0bzFwXjxq8K7Inz0QPgN16EnsQlshWENlti99ULd0pPPgC9Vj MRC1FJZvgC1UEgNB5/X3wMh72XySjbvejFh2JgxyYnO2+gc/910Fy1f/DUK/AnFxSRP2HkYKAi5Ay A1nkFFPlVvLZHDJQxw4PZDVM6/IWhLdR10szkk3VpbK2rFhz+VFvaptbayoyArEpMhzHHNLX4tSok 49f9Ab7A==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1lVig5-003WFx-6L; Sun, 11 Apr 2021 22:35:44 +0000 Date: Sun, 11 Apr 2021 23:35:37 +0100 From: Matthew Wilcox To: Arnd Bergmann Subject: Re: Bogus struct page layout on 32-bit Message-ID: <20210411223537.GF2531743@casper.infradead.org> References: <20210409185105.188284-3-willy@infradead.org> <202104100656.N7EVvkNZ-lkp@intel.com> <20210410024313.GX2531743@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kbuild-all@lists.01.org, kernel test robot , clang-built-linux , Jesper Dangaard Brouer , Linux Kernel Mailing List , Linux-MM , Paul Mackerras , Linux FS-devel Mailing List , linuxppc-dev , "David S. Miller" , Linux ARM Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Sat, Apr 10, 2021 at 09:10:47PM +0200, Arnd Bergmann wrote: > On Sat, Apr 10, 2021 at 4:44 AM Matthew Wilcox wrote: > > + dma_addr_t dma_addr __packed; > > }; > > struct { /* slab, slob and slub */ > > union { > > > > but I don't know if GCC is smart enough to realise that dma_addr is now > > on an 8 byte boundary and it can use a normal instruction to access it, > > or whether it'll do something daft like use byte loads to access it. > > > > We could also do: > > > > + dma_addr_t dma_addr __packed __aligned(sizeof(void *)); > > > > and I see pahole, at least sees this correctly: > > > > struct { > > long unsigned int _page_pool_pad; /* 4 4 */ > > dma_addr_t dma_addr __attribute__((__aligned__(4))); /* 8 8 */ > > } __attribute__((__packed__)) __attribute__((__aligned__(4))); > > > > This presumably affects any 32-bit architecture with a 64-bit phys_addr_t > > / dma_addr_t. Advice, please? > > I've tried out what gcc would make of this: https://godbolt.org/z/aTEbxxbG3 > > struct page { > short a; > struct { > short b; > long long c __attribute__((packed, aligned(2))); > } __attribute__((packed)); > } __attribute__((aligned(8))); > > In this structure, 'c' is clearly aligned to eight bytes, and gcc does > realize that > it is safe to use the 'ldrd' instruction for 32-bit arm, which is forbidden on > struct members with less than 4 byte alignment. However, it also complains > that passing a pointer to 'c' into a function that expects a 'long long' is not > allowed because alignof(c) is only '2' here. > > (I used 'short' here because I having a 64-bit member misaligned by four > bytes wouldn't make a difference to the instructions on Arm, or any other > 32-bit architecture I can think of, regardless of the ABI requirements). So ... we could do this: +++ b/include/linux/types.h @@ -140,7 +140,7 @@ typedef u64 blkcnt_t; * so they don't care about the size of the actual bus addresses. */ #ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT -typedef u64 dma_addr_t; +typedef u64 __attribute__((aligned(sizeof(void *)))) dma_addr_t; #else typedef u32 dma_addr_t; #endif but I'm a little scared that this might have unintended consequences. And Jesper points out that a big-endian 64-bit dma_addr_t can impersonate a PageTail page, and we should solve that problem while we're at it. So I don't think we should do this, but thought I should mention it as a possibility.