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Sun, 11 Apr 2021 09:43:18 +0000 (UTC) Received: from carbon (unknown [10.36.110.19]) by smtp.corp.redhat.com (Postfix) with ESMTP id B66045D9D3; Sun, 11 Apr 2021 09:43:10 +0000 (UTC) Date: Sun, 11 Apr 2021 11:43:07 +0200 From: Jesper Dangaard Brouer To: "Matthew Wilcox (Oracle)" 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 , brouer@redhat.com, Christoph Hellwig Subject: Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems Message-ID: <20210411114307.5087f958@carbon> In-Reply-To: <20210410205246.507048-2-willy@infradead.org> References: <20210410205246.507048-1-willy@infradead.org> <20210410205246.507048-2-willy@infradead.org> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210411_024330_532890_3203F2CF X-CRM114-Status: GOOD ( 41.37 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Sat, 10 Apr 2021 21:52:45 +0100 "Matthew Wilcox (Oracle)" wrote: > 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', so it would have to be cleared on free. 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. > > Fixes: c25fff7171be ("mm: add dma_addr_t to struct page") > Signed-off-by: Matthew Wilcox (Oracle) > --- > include/linux/mm_types.h | 38 ++++++++++++++++++++++++++------------ > 1 file changed, 26 insertions(+), 12 deletions(-) > > 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; The original intend of placing member @dma_addr here is that it overlap with @LRU (type struct list_head) which contains two pointers. Thus, in case of CONFIG_ARCH_DMA_ADDR_T_64BIT=y on 32-bit architectures it would use both pointers. Thinking more about this, this design is flawed as bit 0 of the first word is used for compound pages (see PageTail and @compound_head), is reserved. We knew DMA addresses were aligned, thus we though this satisfied that need. BUT for DMA_ADDR_T_64BIT=y on 32-bit arch the first word will contain the "upper" part of the DMA addr, which I don't think gives this guarantee. I guess, nobody are using this combination?!? I though we added this to satisfy TI (Texas Instrument) driver cpsw (code in drivers/net/ethernet/ti/cpsw*). Thus, I assumed it was in use? > + unsigned long _pp_flags; > + unsigned long pp_magic; > + unsigned long xmi; Matteo notice, I think intent is we can store xdp_mem_info in @xmi. > + unsigned long _pp_mapping_pad; > + dma_addr_t dma_addr; /* might be one or two words */ > }; 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. [1] https://lore.kernel.org/lkml/20210410082158.79ad09a6@carbon/ -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer /* * Return true only if the page has been allocated with * ALLOC_NO_WATERMARKS and the low watermark was not * met implying that the system is under some pressure. */ static inline bool page_is_pfmemalloc(const struct page *page) { /* * Page index cannot be this large so this must be * a pfmemalloc page. */ return page->index == -1UL; } /* * Only to be called by the page allocator on a freshly allocated * page. */ static inline void set_page_pfmemalloc(struct page *page) { page->index = -1UL; } static inline void clear_page_pfmemalloc(struct page *page) { page->index = 0; } _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel