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DIR:OUT; SFP:1101; X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 10 Jun 2021 23:17:19.9804 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: d7eb77b0-f24b-484f-e4df-08d92c65e563 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a; Ip=[216.228.112.34]; Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: BN8NAM11FT065.eop-nam11.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN6PR12MB1698 X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: rcampbell@nvidia.com, willy@infradead.org, linux-doc@vger.kernel.org, nouveau@lists.freedesktop.org, hughd@google.com, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, hch@infradead.org, linux-mm@kvack.org, shakeelb@google.com, bskeggs@redhat.com, jgg@nvidia.com, jhubbard@nvidia.com, akpm@linux-foundation.org, Christoph Hellwig Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Friday, 11 June 2021 9:04:19 AM AEST Peter Xu wrote: > External email: Use caution opening links or attachments > > > On Fri, Jun 11, 2021 at 12:21:26AM +1000, Alistair Popple wrote: > > > Hmm, the thing is.. to me FOLL_SPLIT_PMD should have similar effect to explicit > > > call split_huge_pmd_address(), afaict. Since both of them use __split_huge_pmd() > > > internally which will generate that unwanted CLEAR notify. > > > > Agree that gup calls __split_huge_pmd() via split_huge_pmd_address() > > which will always CLEAR. However gup only calls split_huge_pmd_address() if it > > finds a thp pmd. In follow_pmd_mask() we have: > > > > if (likely(!pmd_trans_huge(pmdval))) > > return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap); > > > > So I don't think we have a problem here. > > Sorry I didn't follow here.. We do FOLL_SPLIT_PMD after this check, right? I > mean, if it's a thp for the current mm, afaict pmd_trans_huge() should return > true above, so we'll skip follow_page_pte(); then we'll check FOLL_SPLIT_PMD > and do the split, then the CLEAR notify. Hmm.. Did I miss something? That seems correct - if the thp is not mapped with a pmd we won't split and we won't CLEAR. If there is a thp pmd we will split and CLEAR, but in that case it is fine - we will retry, but the retry will won't CLEAR because the pmd has already been split. The issue arises with doing it unconditionally in make device exclusive is that you *always* CLEAR even if there is no thp pmd to split. Or at least that's my understanding, please let me know if it doesn't make sense. - Alistair > -- > Peter Xu >