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=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 864D3C433ED for ; Wed, 14 Apr 2021 11:51:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6A111611F2 for ; Wed, 14 Apr 2021 11:51:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1343535AbhDNLwC (ORCPT ); Wed, 14 Apr 2021 07:52:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55942 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233595AbhDNLwC (ORCPT ); Wed, 14 Apr 2021 07:52:02 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 02765C061574; Wed, 14 Apr 2021 04:51:40 -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=6j2YFPpldChGlEhTkgAYG/8AUFiAqXWlLNjztkCHHPQ=; b=J5BS1O4q7ajQjzXd/LDJSC/n5W nLaatbwhpGPw27uZHO96g6+/tyMJhvL6qzIDuTqVYDpLZWsiE1eH4gs+RGnwjwtZofj11rAp/QNXo 5zG9CPBgECBICOpGLmaKDiNFg88n/0HpOy3h47nUCcA+napEcZ6cGJeegmGXvIMAF1IlI0QKv3yBn t6pmOrCaFDoZvjKu2kkCNcN34EEDw6FAeKgqXUk9Y5TV1rcrTtUeqHGfKDG51wcCgKv9JHy4zWIOT ccnhwj9yHyFetqx/J/jBsogxkFb+x3kADp7eIqy6wSJt5ccTYHYk9f87uwXMNh3c7s66KKsbIgnZx RoTlxVDw==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1lWe2m-0073tL-Rj; Wed, 14 Apr 2021 11:51:01 +0000 Date: Wed, 14 Apr 2021 12:50:52 +0100 From: Matthew Wilcox To: Jesper Dangaard Brouer Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, netdev@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org, Ilias Apalodimas , Matteo Croce , Grygorii Strashko , Arnd Bergmann , Christoph Hellwig Subject: Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems Message-ID: <20210414115052.GS2531743@casper.infradead.org> References: <20210410205246.507048-1-willy@infradead.org> <20210410205246.507048-2-willy@infradead.org> <20210411114307.5087f958@carbon> <20210411103318.GC2531743@casper.infradead.org> <20210412011532.GG2531743@casper.infradead.org> <20210414101044.19da09df@carbon> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210414101044.19da09df@carbon> Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org On Wed, Apr 14, 2021 at 10:10:44AM +0200, Jesper Dangaard Brouer wrote: > Yes, indeed! - And very frustrating. It's keeping me up at night. > I'm dreaming about 32 vs 64 bit data structures. My fitbit stats tell > me that I don't sleep well with these kind of dreams ;-) Then you're going to love this ... even with the latest patch, there's still a problem. Because dma_addr_t is still 64-bit aligned _as a type_, that forces the union to be 64-bit aligned (as we already knew and worked around), but what I'd forgotten is that forces the entirety of struct page to be 64-bit aligned. Which means ... /* size: 40, cachelines: 1, members: 4 */ /* padding: 4 */ /* forced alignments: 1 */ /* last cacheline: 40 bytes */ } __attribute__((__aligned__(8))); .. that we still have a hole! It's just moved from being at offset 4 to being at offset 36. > That said, I think we need to have a quicker fix for the immediate > issue with 64-bit bit dma_addr on 32-bit arch and the misalignment hole > it leaves[3] in struct page. In[3] you mention ppc32, does it only > happens on certain 32-bit archs? AFAICT it happens on mips32, ppc32, arm32 and arc. It doesn't happen on x86-32 because dma_addr_t is 32-bit aligned. Doing this fixes it: +++ 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 > I'm seriously considering removing page_pool's support for doing/keeping > DMA-mappings on 32-bit arch's. AFAIK only a single driver use this. ... if you're going to do that, then we don't need to do this.