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Mon, 23 Mar 2020 17:33:49 +0000 Date: Mon, 23 Mar 2020 14:33:45 -0300 From: Jason Gunthorpe To: Christoph Hellwig Cc: Jerome Glisse , Ralph Campbell , Felix.Kuehling@amd.com, linux-mm@kvack.org, John Hubbard , dri-devel@lists.freedesktop.org, amd-gfx@lists.freedesktop.org, Philip Yang Subject: Re: [PATCH hmm 5/6] mm/hmm: remove the CONFIG_TRANSPARENT_HUGEPAGE #ifdef Message-ID: <20200323173345.GB13183@mellanox.com> References: <20200320164905.21722-1-jgg@ziepe.ca> <20200320164905.21722-6-jgg@ziepe.ca> <20200321084317.GE28695@lst.de> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200321084317.GE28695@lst.de> User-Agent: Mutt/1.9.4 (2018-02-28) X-ClientProxiedBy: MN2PR16CA0012.namprd16.prod.outlook.com (2603:10b6:208:134::25) To VI1PR05MB4141.eurprd05.prod.outlook.com (2603:10a6:803:44::15) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from mlx.ziepe.ca (142.68.57.212) by MN2PR16CA0012.namprd16.prod.outlook.com (2603:10b6:208:134::25) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2835.20 via Frontend Transport; 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X-Microsoft-Antispam-Message-Info: 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 X-MS-Exchange-AntiSpam-MessageData: N+VMtfLiiC2kXHFlAWy4B+T9lmwA40UbNwOON8OWCB46ihbmxakfO0q2lJbmMMGLigDkfT+542z3QJRoXk3FFXFfhg69J1LUq7MFyXDWb5s/WbtjNS9ZMKY1MaTffpLF1FOvCywd7/ME9fLcbm8fAg== X-OriginatorOrg: Mellanox.com X-MS-Exchange-CrossTenant-Network-Message-Id: 1d8aa265-3732-455f-40af-08d7cf5058ae X-MS-Exchange-CrossTenant-OriginalArrivalTime: 23 Mar 2020 17:33:49.1001 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: a652971c-7d2e-4d9b-a6a4-d149256f461b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: rCKwzLgKxM9K0fRP+84pUtpCjPR6RKqlWthdrvSZxmC/MKZpoCH/MFr32BEcLaZheDh2orGDkaRFsM+acKGEtw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: VI1PR05MB4448 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: On Sat, Mar 21, 2020 at 09:43:17AM +0100, Christoph Hellwig wrote: > On Fri, Mar 20, 2020 at 01:49:04PM -0300, Jason Gunthorpe wrote: > > From: Jason Gunthorpe > > > > This code can be compiled when CONFIG_TRANSPARENT_HUGEPAGE is off, so > > remove the ifdef. > > It can compile, but will the compiler optimize it away? Yes, the enclosing conditional: if (pmd_devmap(pmd) || pmd_trans_huge(pmd)) { is statically false if !CONFIG_TRANSPARENT_HUGEPAGE This is proven today, as the fallback stub is a function prototype with no implementation: -int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr, - unsigned long end, uint64_t *pfns, pmd_t pmd); If the compiler wasn't optimizing the above branch we'd get link failures. Jason