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=-6.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no 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 DA5E1C433B4 for ; Sat, 10 Apr 2021 19:11:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A03396113A for ; Sat, 10 Apr 2021 19:11:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234911AbhDJTLT (ORCPT ); Sat, 10 Apr 2021 15:11:19 -0400 Received: from mail.kernel.org ([198.145.29.99]:45068 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234768AbhDJTLS (ORCPT ); Sat, 10 Apr 2021 15:11:18 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id BC6966113A; Sat, 10 Apr 2021 19:11:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1618081863; bh=rBfw+r2um68UpFbzVkxVlHHyJ+hgFPf/zBrFYQw7TNI=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=CIhhz328zzet5a/BvXZzSs9qZeOvsRmdTIQX8oHvPt4oPq31f6SdMiRQlV3wwxeNa GmirfKbnBbXy13rC5mPC5X4IYTxOrbfSp201pVc2ivwXNFWXhMjpUUd6Z3P6GdAypc hXQbk3jNgzXE4abVKr5LlTbMx3yus3tJYykOkPF4g5QUy/4sEmN3QnSl1XUTyImSG8 +nrIDp4z3hEGajjGuvQYOwmAvld9GZf6ZqRXKlZ6kJNOxc2eYZ8HIjAVZ3TijeONug 5j3gyVLCEPaB6nGAjThA9V1IH5nKdc2cpKmOF0OJJIEPxgrvw2FaG4CSqtEAVnEU7R BXNDi+Akq/Lhw== Received: by mail-ot1-f47.google.com with SMTP id k14-20020a9d7dce0000b02901b866632f29so8973160otn.1; Sat, 10 Apr 2021 12:11:03 -0700 (PDT) X-Gm-Message-State: AOAM531Gu7ZroMfzZPVK+zswSeVKSRAt6gNScNWY7toYwzv0pgTbqngs YoYRoVagkOv02V8N9BihJZYRoDdswmEDZttOAdg= X-Google-Smtp-Source: ABdhPJy5fKRN+b0N+vdm7HuaN0voD9Ksx0FbjR4NiXXEFlqO3Rbtb6TftW/JNl1MSl+hXOUABL/gf3HkAiDbD0wFK7g= X-Received: by 2002:a05:6830:148c:: with SMTP id s12mr17843523otq.251.1618081863076; Sat, 10 Apr 2021 12:11:03 -0700 (PDT) MIME-Version: 1.0 References: <20210409185105.188284-3-willy@infradead.org> <202104100656.N7EVvkNZ-lkp@intel.com> <20210410024313.GX2531743@casper.infradead.org> In-Reply-To: <20210410024313.GX2531743@casper.infradead.org> From: Arnd Bergmann Date: Sat, 10 Apr 2021 21:10:47 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: Bogus struct page layout on 32-bit To: Matthew Wilcox Cc: kernel test robot , Linux-MM , kbuild-all@lists.01.org, clang-built-linux , Linux Kernel Mailing List , Linux FS-devel Mailing List , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , linuxppc-dev , Linux ARM , Jesper Dangaard Brouer , "David S. Miller" Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org 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). Arnd