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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 00E15C433B4 for ; Sat, 10 Apr 2021 02:44:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C7ABB610A2 for ; Sat, 10 Apr 2021 02:44:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233977AbhDJCo3 (ORCPT ); Fri, 9 Apr 2021 22:44:29 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47002 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229665AbhDJCo0 (ORCPT ); Fri, 9 Apr 2021 22:44:26 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3AE7CC061762; Fri, 9 Apr 2021 19:44:13 -0700 (PDT) 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=kSBfGQGdldHOGZqey2pG91Lm4sBchHdpUdkbO6By1YI=; b=Exz1xebH0z4i/y6qs7XFO141Qk eRdSApUIFaSir1NyVFycHEYEbOXEp95qDIa230dForIUOhOrcL8b410X67OZZNpDdPq7jmGPKucr7 Jm/mNgBVC4BZbhVGO7ZiGnxsB72BAayKt8FYPWcUkpAAo/QYX9jaHmqnmcaOOEaZUAvoCv7+YoC+d azojKss4OD+27TdGNupRux2mmMjSbuKCdgisakoI1PTwZoivDI2KZGt9qSU6szreGgStQJ6HXtug+ 4dyr/8D2BnK5laEKba2tcv99gMP977IVwZBCOGjK+KP98CtMI/JYoGJ42Sjy4kn89qoYCE05lmJgV 2L/4kfhQ==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1lV3ab-001Esh-RM; Sat, 10 Apr 2021 02:43:27 +0000 Date: Sat, 10 Apr 2021 03:43:13 +0100 From: Matthew Wilcox To: kernel test robot Cc: linux-mm@kvack.org, kbuild-all@lists.01.org, clang-built-linux@googlegroups.com, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org, Jesper Dangaard Brouer , "David S. Miller" Subject: Bogus struct page layout on 32-bit Message-ID: <20210410024313.GX2531743@casper.infradead.org> References: <20210409185105.188284-3-willy@infradead.org> <202104100656.N7EVvkNZ-lkp@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <202104100656.N7EVvkNZ-lkp@intel.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Apr 10, 2021 at 06:45:35AM +0800, kernel test robot wrote: > >> include/linux/mm_types.h:274:1: error: static_assert failed due to requirement '__builtin_offsetof(struct page, lru) == __builtin_offsetof(struct folio, lru)' "offsetof(struct page, lru) == offsetof(struct folio, lru)" > FOLIO_MATCH(lru, lru); > include/linux/mm_types.h:272:2: note: expanded from macro 'FOLIO_MATCH' > static_assert(offsetof(struct page, pg) == offsetof(struct folio, fl)) Well, this is interesting. pahole reports: struct page { long unsigned int flags; /* 0 4 */ /* XXX 4 bytes hole, try to pack */ union { struct { struct list_head lru; /* 8 8 */ ... struct folio { union { struct { long unsigned int flags; /* 0 4 */ struct list_head lru; /* 4 8 */ so this assert has absolutely done its job. But why has this assert triggered? Why is struct page layout not what we thought it was? Turns out it's the dma_addr added in 2019 by commit c25fff7171be ("mm: add dma_addr_t to struct page"). On this particular config, it's 64-bit, and ppc32 requires alignment to 64-bit. So the whole union gets moved out by 4 bytes. Unfortunately, we can't just fix this by putting an 'unsigned long pad' in front of it. It still aligns the entire union to 8 bytes, and then it skips another 4 bytes after the pad. We can fix it like this ... +++ b/include/linux/mm_types.h @@ -96,11 +96,12 @@ struct page { unsigned long private; }; struct { /* page_pool used by netstack */ + unsigned long _page_pool_pad; /** * @dma_addr: might require a 64-bit value even on * 32-bit architectures. */ - dma_addr_t dma_addr; + 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?