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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, 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 1443DC43462 for ; Mon, 12 Apr 2021 18:24:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D80D561350 for ; Mon, 12 Apr 2021 18:24:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244773AbhDLSYi (ORCPT ); Mon, 12 Apr 2021 14:24:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50056 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229666AbhDLSYh (ORCPT ); Mon, 12 Apr 2021 14:24:37 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 30363C061574; Mon, 12 Apr 2021 11:24:19 -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=djSd+P2nuqDFfB/9mLdQPbwoAlNn60FgiY/LPsvCEKc=; b=Wvk8Hyjsbg3T98nGoi7OKr8yII 5eC9VyxYVI7KymL749Vz/DrsJLq7orZiLaP2l9sv55pYQ8loJ4skAMQ7j4/oQOsXj6TNQPZlnRLxW +PLlXP5pJT95owrhGgU+kQbom/gfMiREOlixdQequzZ8aP4D2tLKew4x5klJ1mxCnT14rqDqgBQbf K/SVD5viM3hKW7ErpzwIdyDChv38HYXREMyF0gj4pUs7MXHtnj7oyw23bo1QF2GpG2srIHZakmX7S Vlxfoot5VdeBFdbjlhLKlrutfkIGCJDftBRvPZ5E5V8+cFOiSWFZrVPgrfqumahwHkusIApkof9zK 3IN6MiNA==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1lW1E2-004jlZ-2K; Mon, 12 Apr 2021 18:24:03 +0000 Date: Mon, 12 Apr 2021 19:23:54 +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 , Ivan Khoronzhuk , Grygorii Strashko , Arnd Bergmann , Christoph Hellwig Subject: Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems Message-ID: <20210412182354.GN2531743@casper.infradead.org> References: <20210410205246.507048-1-willy@infradead.org> <20210410205246.507048-2-willy@infradead.org> <20210411114307.5087f958@carbon> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210411114307.5087f958@carbon> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Apr 11, 2021 at 11:43:07AM +0200, Jesper Dangaard Brouer wrote: > Could you explain your intent here? > I worry about @index. > > As I mentioned in other thread[1] netstack use page_is_pfmemalloc() > (code copy-pasted below signature) which imply that the member @index > have to be kept intact. In above, I'm unsure @index is untouched. Well, I tried three different approaches. Here's the one I hated the least. From: "Matthew Wilcox (Oracle)" Date: Sat, 10 Apr 2021 16:12:06 -0400 Subject: [PATCH] mm: Fix struct page layout on 32-bit systems 32-bit architectures which expect 8-byte alignment for 8-byte integers and need 64-bit DMA addresses (arc, arm, mips, ppc) had their struct page inadvertently expanded in 2019. When the dma_addr_t was added, it forced the alignment of the union to 8 bytes, which inserted a 4 byte gap between 'flags' and the union. We could fix this by telling the compiler to use a smaller alignment for the dma_addr, but that seems a little fragile. Instead, move the 'flags' into the union. That causes dma_addr to shift into the same bits as 'mapping', which causes problems with page_mapping() called from set_page_dirty() in the munmap path. To avoid this, insert three words of padding and use the same bits as ->index and ->private, neither of which have to be cleared on free. However, page->index is currently used to indicate page_is_pfmemalloc. Move that information to bit 1 of page->lru (aka compound_head). This has the same properties; it will be overwritten by callers who do not care about pfmemalloc (as opposed to using a bit in page->flags). Fixes: c25fff7171be ("mm: add dma_addr_t to struct page") Signed-off-by: Matthew Wilcox (Oracle) --- include/linux/mm.h | 12 +++++++----- include/linux/mm_types.h | 38 ++++++++++++++++++++++++++------------ 2 files changed, 33 insertions(+), 17 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index b58c73e50da0..23cca0eaa9da 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1668,10 +1668,12 @@ struct address_space *page_mapping(struct page *page); static inline bool page_is_pfmemalloc(const struct page *page) { /* - * Page index cannot be this large so this must be - * a pfmemalloc page. + * This is not a tail page; compound_head of a head page is unused + * at return from the page allocator, and will be overwritten + * by callers who do not care whether the page came from the + * reserves. */ - return page->index == -1UL; + return page->compound_head & 2; } /* @@ -1680,12 +1682,12 @@ static inline bool page_is_pfmemalloc(const struct page *page) */ static inline void set_page_pfmemalloc(struct page *page) { - page->index = -1UL; + page->compound_head = 2; } static inline void clear_page_pfmemalloc(struct page *page) { - page->index = 0; + page->compound_head = 0; } /* diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 6613b26a8894..45c563e9b50e 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -68,16 +68,22 @@ struct mem_cgroup; #endif struct page { - unsigned long flags; /* Atomic flags, some possibly - * updated asynchronously */ /* - * Five words (20/40 bytes) are available in this union. - * WARNING: bit 0 of the first word is used for PageTail(). That - * means the other users of this union MUST NOT use the bit to + * This union is six words (24 / 48 bytes) in size. + * The first word is reserved for atomic flags, often updated + * asynchronously. Use the PageFoo() macros to access it. Some + * of the flags can be reused for your own purposes, but the + * word as a whole often contains other information and overwriting + * it will cause functions like page_zone() and page_node() to stop + * working correctly. + * + * Bit 0 of the second word is used for PageTail(). That + * means the other users of this union MUST leave the bit zero to * avoid collision and false-positive PageTail(). */ union { struct { /* Page cache and anonymous pages */ + unsigned long flags; /** * @lru: Pageout list, eg. active_list protected by * lruvec->lru_lock. Sometimes used as a generic list @@ -96,13 +102,14 @@ struct page { unsigned long private; }; struct { /* page_pool used by netstack */ - /** - * @dma_addr: might require a 64-bit value even on - * 32-bit architectures. - */ - dma_addr_t dma_addr; + unsigned long _pp_flags; + unsigned long pp_magic; + unsigned long xmi; + unsigned long _pp_mapping_pad; + dma_addr_t dma_addr; /* might be one or two words */ }; struct { /* slab, slob and slub */ + unsigned long _slab_flags; union { struct list_head slab_list; struct { /* Partial pages */ @@ -130,6 +137,7 @@ struct page { }; }; struct { /* Tail pages of compound page */ + unsigned long _t1_flags; unsigned long compound_head; /* Bit zero is set */ /* First tail page only */ @@ -139,12 +147,14 @@ struct page { unsigned int compound_nr; /* 1 << compound_order */ }; struct { /* Second tail page of compound page */ + unsigned long _t2_flags; unsigned long _compound_pad_1; /* compound_head */ atomic_t hpage_pinned_refcount; /* For both global and memcg */ struct list_head deferred_list; }; struct { /* Page table pages */ + unsigned long _pt_flags; unsigned long _pt_pad_1; /* compound_head */ pgtable_t pmd_huge_pte; /* protected by page->ptl */ unsigned long _pt_pad_2; /* mapping */ @@ -159,6 +169,7 @@ struct page { #endif }; struct { /* ZONE_DEVICE pages */ + unsigned long _zd_flags; /** @pgmap: Points to the hosting device page map. */ struct dev_pagemap *pgmap; void *zone_device_data; @@ -174,8 +185,11 @@ struct page { */ }; - /** @rcu_head: You can use this to free a page by RCU. */ - struct rcu_head rcu_head; + struct { + unsigned long _rcu_flags; + /** @rcu_head: You can use this to free a page by RCU. */ + struct rcu_head rcu_head; + }; }; union { /* This union is 4 bytes in size. */ -- 2.30.2