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:   Wed, 15 Jun 2022 12:56:59 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Alex Williamson <alex.williamson@redhat.com>,
        akpm@linux-foundation.org, minchan@kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, paulmck@kernel.org,
        jhubbard@nvidia.com, joaodias@google.com
Subject: Re: [PATCH] mm: Re-allow pinning of zero pfns
Message-ID: <20220615155659.GA7684@nvidia.com>
References: <165490039431.944052.12458624139225785964.stgit@omen>
 <bb120b4a-e6f6-de81-35f0-9803acf9b0be@redhat.com>
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <bb120b4a-e6f6-de81-35f0-9803acf9b0be@redhat.com>
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?NVHqJsUWzwGVWf8dXU97r9XwYZ6KEIgIddkoaLsEdFkXQV51HVswy54dHJ+J?=
 =?us-ascii?Q?9OC7CYUs7IKLTKhXnl3CI7ic6m20J4BeUvUBpS869HCQZQX7/887n2wzlXFH?=
 =?us-ascii?Q?jIRCKHBeRlppzwKHAh6pn6yuNLQnQN1Vm1O252JZAPcBtvY16U4d/5Dlckis?=
 =?us-ascii?Q?lof7rl3eUrDJlvbbTwCHX7AsGNppt8tlm7OI677e8hbZVJVD0lObHTSBjMS+?=
 =?us-ascii?Q?OPwwLqiNudetUcCDIS7DiADGRw7TLCJvcvB9C0YojY/AQYzZvQAyS/9ZaY91?=
 =?us-ascii?Q?XMu6aTeMA8rEHPugyUCGScuH9WhnsogI/VzpQdh/wczeuT4cKYKP+J8RjR+n?=
 =?us-ascii?Q?u6zcz4NFN9xCjWTwdqqJOF2Zu9zSKm0WfhUO08lqpBn04AIhaZ45MxZVLoW7?=
 =?us-ascii?Q?XQuFn03WoZ2QZgOuhowbcE8d1P2DIzROXKE+K9pVHYNAGWkjr22XYEP3EAue?=
 =?us-ascii?Q?X+UMgB5A1Nmj7K1y8xB0WP85pUyAMO3SWrtK7Yc+EHYwdFveBHXeZApTPuWf?=
 =?us-ascii?Q?j6DVK+2tICZYOBGLdvHCyEPigOfpkUSlQQslCli1bHZ7ghcwA1D1PKW5agCL?=
 =?us-ascii?Q?cqhlS4qIKfLuV7rt0PKVAlUWypUz+5e27BrpueCSmgNivXyfSqHv10bgdrEJ?=
 =?us-ascii?Q?5VBe9IOqncenICQPsMV0NzyFRumVUlSHsaSwmS78DFJ3rg7eTyKXrO2OqE2R?=
 =?us-ascii?Q?+94/ptMS721irpzJ4q5SAivtlgITirjdEnQ4LuthihAvQWuzyN4VrMtTADhp?=
 =?us-ascii?Q?tXbRNAf2/X9GBAINKZ6CNCPORhJ/K1inOVLjmnH0XeFfEDszeledfiIzy+hS?=
 =?us-ascii?Q?96OsCV3Bjg5uIhAJfGTO0vgrfP67Vs32lVusRuDRin22mJPq2Y9r0GFvxgZ6?=
 =?us-ascii?Q?VJlAzGAo+PN7yXGYSbNgAKI8nznN/QyZy+ZSxfOSBPZy32RJHC4KlsxTP1Qv?=
 =?us-ascii?Q?lbj4cOf2P5eZX/xHvQQaGW88tTV5X70v6SEYwpXvKoIXm4uoE5kKzWI8WBCq?=
 =?us-ascii?Q?qfIlQc6doT2aEc7YKiViz+dKSNJeiqT6IqthY4eOsdUlxlbb7vUxgD65AIYw?=
 =?us-ascii?Q?12sSQDEjP7qlw5WyjWif12wRMeIuBHy7U793B4f/hFnXzHGgD5DaALF6cypN?=
 =?us-ascii?Q?3hzy2DkdGn0UXoER5myZkktY/cn/UQ2OTOq6ucBSUn9oqmy0n/bzVW/0PD1L?=
 =?us-ascii?Q?6wr7IB/A9S8s8TwXTnr9ta9JovKdztYcIn1DYU9h4nLEvT4hMLjUhTjXpRj0?=
 =?us-ascii?Q?4YARSzZnrdY4CkXct5ZZmZYwgFIocjD3qkxIXhrKbULtC90LStK91U2U1Igc?=
 =?us-ascii?Q?xPlbCS/jtBJwXiva0Q4sy98vYMGrClBiOezvkzWS8B1gIR+4/jf/eucS+hfz?=
 =?us-ascii?Q?c8DgXVVqOAYsCJUXleZk2kOOnar+Kd/AuEZi1Pdq8EVepYoTE9AH0k7NLGNo?=
 =?us-ascii?Q?hvZX39CfT2CQcGZTm9lGhxl2SecThPsqO7VRL9GlLMv032+gBiym/Ym9R2TW?=
 =?us-ascii?Q?7hsPcNBlkTID12S4on33YP/m7T/iuWXgooI4657H5IawTQYDv+Y5l+7Z1EPI?=
 =?us-ascii?Q?yUx8TkQ/gz/8FNsLX8D9S+RUz2qL+bbBqpBV4nH94+CU23a41HL6/T4DkY/E?=
 =?us-ascii?Q?FhefK/n+NVd3PD4lIQ5pTFMLSEZpX2SdfGCema2KgGqnCfPo34B/T0RAyi5D?=
 =?us-ascii?Q?6NBwuQKAcuvXB0JqKtA9OPeyEVtEcRvJjUxv+6ZDkxZmhUSKQRyPi38K6K81?=
 =?us-ascii?Q?hsq6nh8a2Q=3D=3D?=
X-OriginatorOrg: Nvidia.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 7253f353-db00-4d9a-db94-08da4ee7af96
X-MS-Exchange-CrossTenant-AuthSource: CH2PR12MB4181.namprd12.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 15 Jun 2022 15:57:02.0094
 (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: FGIeihKxuFqYNa9GNkTk67lg7N8H1NEEe1Zye1PvSII9NSnP8OQEeOulH8ZEjuYd
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR12MB3964
X-Spam-Status: No, score=-2.3 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 Sat, Jun 11, 2022 at 08:29:47PM +0200, David Hildenbrand wrote:
> On 11.06.22 00:35, Alex Williamson wrote:
> > The commit referenced below subtly and inadvertently changed the logic
> > to disallow pinning of zero pfns.  This breaks device assignment with
> > vfio and potentially various other users of gup.  Exclude the zero page
> > test from the negation.
> 
> I wonder which setups can reliably work with a long-term pin on a shared
> zeropage. In a MAP_PRIVATE mapping, any write access via the page tables
> will end up replacing the shared zeropage with an anonymous page.
> Something similar should apply in MAP_SHARED mappings, when lazily
> allocating disk blocks.
> 
> In the future, we might trigger unsharing when taking a R/O pin for the
> shared zeropage, just like we do as of now upstream for shared anonymous
> pages (!PageAnonExclusive). And something similar could then be done
> when finding a !anon page in a MAP_SHARED mapping.

I'm also confused how qemu is hitting this and it isn't already a bug?

It is arising because vfio doesn't use FOLL_FORCE|FOLL_WRITE to move
away the zero page in most cases.

And why does Yishai say it causes an infinite loop in the kernel?

Jason