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Thu, 25 Mar 2021 13:05:19 +0000 Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages To: Jason Gunthorpe Cc: =?UTF-8?Q?Thomas_Hellstr=c3=b6m_=28Intel=29?= , David Airlie , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-mm@kvack.org, Andrew Morton References: <08f19e80-d6cb-8858-0c5d-67d2e2723f72@amd.com> <730eb2ff-ba98-2393-6d42-61735e3c6b83@shipmail.org> <20210324231419.GR2356281@nvidia.com> <607ecbeb-e8a5-66e9-6fe2-9a8d22f12bc2@shipmail.org> <15da5784-96ca-25e5-1485-3ce387ee6695@shipmail.org> <20210325113023.GT2356281@nvidia.com> <20210325120103.GV2356281@nvidia.com> <20210325124206.GA599656@nvidia.com> From: =?UTF-8?Q?Christian_K=c3=b6nig?= Message-ID: <00f79bae-75c4-d694-8dc9-35ac21cd1006@amd.com> Date: Thu, 25 Mar 2021 14:05:14 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 In-Reply-To: <20210325124206.GA599656@nvidia.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Originating-IP: [2a02:908:1252:fb60:72d1:2a48:a80e:e149] X-ClientProxiedBy: AM0PR05CA0096.eurprd05.prod.outlook.com (2603:10a6:208:136::36) To MN2PR12MB3775.namprd12.prod.outlook.com (2603:10b6:208:159::19) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [IPv6:2a02:908:1252:fb60:72d1:2a48:a80e:e149] (2a02:908:1252:fb60:72d1:2a48:a80e:e149) by AM0PR05CA0096.eurprd05.prod.outlook.com (2603:10a6:208:136::36) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3977.29 via Frontend Transport; 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identity=mailfrom; envelope-from=""; helo=NAM10-BN7-obe.outbound.protection.outlook.com; client-ip=40.107.92.61 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616677526-833134 Content-Transfer-Encoding: quoted-printable 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: Am 25.03.21 um 13:42 schrieb Jason Gunthorpe: > On Thu, Mar 25, 2021 at 01:09:14PM +0100, Christian K=C3=B6nig wrote: >> Am 25.03.21 um 13:01 schrieb Jason Gunthorpe: >>> On Thu, Mar 25, 2021 at 12:53:15PM +0100, Thomas Hellstr=C3=B6m (Inte= l) wrote: >>> >>>> Nope. The point here was that in this case, to make sure mmap uses t= he >>>> correct VA to give us a reasonable chance of alignement, the driver = might >>>> need to be aware of and do trickery with the huge page-table-entry s= izes >>>> anyway, although I think in most cases a standard helper for this ca= n be >>>> supplied. >>> Of course the driver needs some way to influence the VA mmap uses, >>> gernally it should align to the natural page size of the device >> Well a mmap() needs to be aligned to the page size of the CPU, but not >> necessarily to the one of the device. >> >> So I'm pretty sure the device driver should not be involved in any way= the >> choosing of the VA for the CPU mapping. > No, if the device wants to use huge pages it must influence the mmap > VA or it can't form huge pgaes. No, that's the job of the core MM and not of the individual driver. In other words current->mm->get_unmapped_area should already return a=20 properly aligned VA. Messing with that inside file->f_op->get_unmapped_area is utterly=20 nonsense as far as I can see. It happens to be this way currently, but that is not even remotely good=20 design. Christian. > > It is the same reason why mmap returns 2M stuff these days to make THP > possible > > Jason