Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 18 Mar 2022 20:30:35 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Nadav Amit <namit@vmware.com>, Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
        Liang Zhang <zhangliang5@huawei.com>,
        Pedro Gomes <pedrodemargomes@gmail.com>,
        Oded Gabbay <oded.gabbay@gmail.com>, linux-mm@kvack.org
Subject: Re: [PATCH v2 13/15] mm: support GUP-triggered unsharing of
 anonymous pages
Message-ID: <20220318233035.GA11336@nvidia.com>
References: <20220315104741.63071-1-david@redhat.com>
 <20220315104741.63071-14-david@redhat.com>
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20220315104741.63071-14-david@redhat.com>
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?oHbbC/1m2JAlug6wYIsjNYUgdBdirMmNF5aw3rxybCo+xQAQLWLnR9T7vAdd?=
 =?us-ascii?Q?ffLHvaFJVlN/2OWiEkSpVg/3pAvijFOHGUsL3OebqpEu6TMg+to9IfbuTSDv?=
 =?us-ascii?Q?IYe+ExDLNArsRJn9p/42aW+1JyCVUdO817MEwkpPH+ax1+BDZu/AJno8zibs?=
 =?us-ascii?Q?kcRs1+/QBzs5Q8bM1OmVRQ36ef2a6WGtNrdvNzNu6dlz+0JHtgCCZDHuQCey?=
 =?us-ascii?Q?YXf1TIAYdaLg2OpUZLmQ1KvmfWCD2mQ2pgstvr3D435WOZT7JUV3QnE/b+fE?=
 =?us-ascii?Q?C2nCHjUXCfHqK4RPzwjhSAQelCdbrCnA1esvkPOuLpyECXVdIu/vjPsCyeWS?=
 =?us-ascii?Q?nY+6FoX9MR75oz0gDrrBAb8UI6qt/pvREI8vQDy2KZ5Jr1wJvViHJ5LTDcP1?=
 =?us-ascii?Q?vx6z0L2OT7OeWR8oooZx5K41IU7tCpdnCPiFSoEQFmln0TJCIwcQ9iVBUxEK?=
 =?us-ascii?Q?rNNfkonyNPsN1wl7Jm3FwnS08E0yJ412qgDsdwX+tZgDL1K8iqEC6PGD+LBx?=
 =?us-ascii?Q?YmvAz03PIJWYEVGNqUbV4cZfg19iLFuiNFUlYruoqpFvqeblYAZK+nHqLBLO?=
 =?us-ascii?Q?0uNknfwW4Rnx/4+ERHO6I5QtzF9O2+nav9w3b1LId2eaqQJzI3XbhFt/pWXh?=
 =?us-ascii?Q?Ufm81xOnlXgHxHVxTc7Bkeu7NG7Gg0NAWySbUZA6Nm/Xp5J9W61zRH0i7y24?=
 =?us-ascii?Q?vY5Knbj9xj0XZ5G1/7R42W7k1XRKlUdavrNda0TtaVY913R87Nps5JjyM2V+?=
 =?us-ascii?Q?O+XMy1vpZ0pfjqCr/3d/K9p991TUBUWzCbPBKZc7qHQcWn7Y5tt1qbnzVLKW?=
 =?us-ascii?Q?qsj+mNhBafvBpM+nK8ZQoDHjuivwzSLnyvAI3CvYHZXJP90GeE4+QzfzkOFp?=
 =?us-ascii?Q?l70/vKCNlcANoFBD00I8EFmUC5YbBObDOlEpSbwQfrkwIOZesl8XlQax9rqa?=
 =?us-ascii?Q?ryO5lkJ9xBWea2hgJUe4ejizLPU7Tb929bmACogNljrhIIXJ0yignLKQiCjb?=
 =?us-ascii?Q?+yuDZY0U3gxCR6FXoqmjDY7XzU6Ydhh52tcaRYPMbNe/PvIIGsRRWPlg3u4p?=
 =?us-ascii?Q?3kyO+jiNCqKYo5U6t2R5TGL1C6ny3mKboqLd8tE7GNvqz+Jh891/wVRTlUXX?=
 =?us-ascii?Q?bKR0wHCGA1ly+x/Sm2WT7+4V54sXcT6OlRNpO5qDGIrVhRY24R8kIazp/L5u?=
 =?us-ascii?Q?8EK4cWwr3kkZK628qS37wMvIxjnZNFO38R8Xe60eD5Ztwo3GkKlhKGwvo7Xb?=
 =?us-ascii?Q?bv0ERW8kpgu3/aX3Zet6mvMdP/RRHsWgH8N/K1l0aazPZk6+IdhXAyAIwPi6?=
 =?us-ascii?Q?5B5ZPa0m1s9Z+rumsJ2QAGjMQmpb580HzFsZsMOLK05FRD3jFrglhNXRQcYz?=
 =?us-ascii?Q?hvbrsjNkaHRQ/xvwRuRcyVygG0dzhOw/MPICNFZRwtB1ab519k82e/WJlj5X?=
 =?us-ascii?Q?PyUzyNzsD1QFfkgaV92p64hxrxYSDIKk?=
X-OriginatorOrg: Nvidia.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 6d08d2bb-ad47-43f1-0540-08da09374e1e
X-MS-Exchange-CrossTenant-AuthSource: MN2PR12MB4192.namprd12.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 18 Mar 2022 23:30:37.0279
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: d/87JDeun022Y+l7KNqJo2mZD4g4TYDQhzSsRJo3sZ8yH6u/uBjGmen8hXWzsV9u
X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR12MB3479
X-Spam-Status: No, score=-2.6 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,
        DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FORGED_SPF_HELO,
        RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_PASS,SPF_NONE,
        T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
        lindbergh.monkeyblade.net
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, Mar 15, 2022 at 11:47:39AM +0100, David Hildenbrand wrote:
> Whenever GUP currently ends up taking a R/O pin on an anonymous page that
> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault
> on the page table entry will end up replacing the mapped anonymous page
> due to COW, resulting in the GUP pin no longer being consistent with the
> page actually mapped into the page table.
> 
> The possible ways to deal with this situation are:
>  (1) Ignore and pin -- what we do right now.
>  (2) Fail to pin -- which would be rather surprising to callers and
>      could break user space.
>  (3) Trigger unsharing and pin the now exclusive page -- reliable R/O
>      pins.
> 
> We want to implement 3) because it provides the clearest semantics and
> allows for checking in unpin_user_pages() and friends for possible BUGs:
> when trying to unpin a page that's no longer exclusive, clearly
> something went very wrong and might result in memory corruptions that
> might be hard to debug. So we better have a nice way to spot such
> issues.
> 
> To implement 3), we need a way for GUP to trigger unsharing:
> FAULT_FLAG_UNSHARE. FAULT_FLAG_UNSHARE is only applicable to R/O mapped
> anonymous pages and resembles COW logic during a write fault. However, in
> contrast to a write fault, GUP-triggered unsharing will, for example, still
> maintain the write protection.

Given the way this series has developed you might want to call this
FAULT_FLAG_MAKE_ANON_EXCLUSIVE

Which strikes me as more directly connected to what it is trying to
do.

Jason