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Thu, 15 Apr 2021 22:11:57 +0100 X-MC-Unique: edG-rOXuODirktZxzgM_1A-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) by AcuMS.aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Thu, 15 Apr 2021 22:11:56 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.012; Thu, 15 Apr 2021 22:11:56 +0100 From: David Laight To: 'Matthew Wilcox' , 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 Thread-Topic: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems Thread-Index: AQHXMXYmwdfrgigLI0exh4xFUSZq9Kq0jZ3ggAFYFFmAAC15oA== Date: Thu, 15 Apr 2021 21:11:56 +0000 Message-ID: <5179a01a462f43d6951a65de2a299070@AcuMS.aculab.com> References: <20210410205246.507048-2-willy@infradead.org> <20210411114307.5087f958@carbon> <20210411103318.GC2531743@casper.infradead.org> <20210412011532.GG2531743@casper.infradead.org> <20210414101044.19da09df@carbon> <20210414115052.GS2531743@casper.infradead.org> <20210414211322.3799afd4@carbon> <20210414213556.GY2531743@casper.infradead.org> <20210415200832.32796445@carbon> <20210415182155.GD2531743@casper.infradead.org> In-Reply-To: <20210415182155.GD2531743@casper.infradead.org> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: ACBE840002D3 X-Stat-Signature: m9h4re41qziikdccioo1jrscq5ghyqpk Received-SPF: none (aculab.com>: No applicable sender policy available) receiver=imf17; identity=mailfrom; envelope-from=""; helo=eu-smtp-delivery-151.mimecast.com; client-ip=185.58.86.151 X-HE-DKIM-Result: none/none X-HE-Tag: 1618521119-302868 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: From: Matthew Wilcox > Sent: 15 April 2021 19:22 >=20 > On Thu, Apr 15, 2021 at 08:08:32PM +0200, Jesper Dangaard Brouer wrote: > > +static inline > > +dma_addr_t page_pool_dma_addr_read(dma_addr_t dma_addr) > > +{ > > +=09/* Workaround for storing 64-bit DMA-addr on 32-bit machines in str= uct > > +=09 * page. The page->dma_addr share area with page->compound_head wh= ich > > +=09 * use bit zero to mark compound pages. This is okay, as DMA-addr a= re > > +=09 * aligned pointers which have bit zero cleared. > > +=09 * > > +=09 * In the 32-bit case, page->compound_head is 32-bit. Thus, when > > +=09 * dma_addr_t is 64-bit it will be located in top 32-bit. Solve by > > +=09 * swapping dma_addr 32-bit segments. > > +=09 */ > > +#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT >=20 > #if defined(CONFIG_ARCH_DMA_ADDR_T_64BIT) && defined(__BIG_ENDIAN) > otherwise you'll create the problem on ARM that you're avoiding on PPC ..= . >=20 > I think you want to delete the word '_read' from this function name becau= se > you're using it for both read and write. I think I'd use explicit dma_addr_hi and dma_addr_lo and separate read/write functions just to make absolutely sure nothing picks up the swapped value. Isn't it possible to move the field down one long? This might require an explicit zero - but this is not a common code path - the extra write will be noise. =09David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)