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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: 5IFhDEfTAQ92Q/G9Gll5lN/doAc6Y4NPVWgbEuvSN6YYYf4YuuJ2zYmYX8/XpfhIiwHHOvwlQoUUDQXZfcQWELXuXBFX/EoRs6tlJ7dEY8BWoqP1/xZshufEE29dVM1XUPE2BU1f4Eaxem68s1RU3PQjg/F+vNFo8bGRE3ALK6+Wr9LcdR25U7hniDzJqvguVom4u/f5P60c+Yz44USyZ+7GX0Hd7ZCMsx7GLx+1ZNurIfXE1kPVJFKMLaC5gkSUJMr/c5aXhphDe+7NngS1I0z9N82PK27q7wMbML8QeT0xLsnwRwAt6BwLEABVhqSnpJNW/koWfEHugDGEzWBp7/t7Pt9eejoKi69CGWPmlt0QsFt1rHOZq/T2Sbn+hj9axvICiyFldMPWHfZjg5VmzoeJbDJqVVrknAK0LzYvmbjJUdnCGXf/CI1y0zUFQ80h X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 17 Jun 2026 13:42:45.4065 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 0051b8c9-4878-46fc-f41d-08decc765017 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB3PEPF0000885E.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PR3PR08MB5690 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260617_064252_696956_6B8D4928 X-CRM114-Status: GOOD ( 12.98 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 6/17/26 15:23, Alexandru Elisei wrote: > Hi David, > > On Mon, Jun 15, 2026 at 09:07:50PM +0200, David Hildenbrand wrote: >> On 6/15/26 17:52, Alexandru Elisei wrote: >>> For guest_memfd-only memslots (kvm_memslot_is_gmem_only() is true), the >>> memory provider for the virtual machine is the guest_memfd file, not the >>> userspace mapping. Faults are resolved using the guest_memfd page cache, >>> and the permissions for the secondary MMU mapping depends exclusively on >>> the memslot (i.e, if the memslot is read-only). How userspace happens to >>> have the memory mmaped at fault time, or even if the memory is mapped at >>> all into userspace, is not taken into consideration. >>> >>> guest_memfd memory is not evictable, is not movable and there's no backing >>> storage. Once memory is allocated for an offset in guest_memfd file, the >>> offset will not change, and that memory is not freed unless userspace >>> explicitly punches a hole in the file. As a result, memory reclaim, page >>> migration, page aging and dirty page tracking for the userspace mapping >>> serve little purpose. >> >> I don't think any of that is relevant for the patch at hand? >> >> The thing is: invalidation (truncation, later migration, for any other reason) >> is driven through guest_memfd notifications, not through unrelated page tables. >> >> If we don't lookup pages for the KVM MMU through the page table, then there is >> also no need for MMU notifiers. It's all guest_memfd only. >> >> Or am I missing something? > > My thinking was that, because guest_memfd is not evictable, there is no need to > do page ageing, which would require that secondary MMU mappings be made old. Not really. The KVM MMU did not obtain the folios through the page tables, but directly through guest_memfd. Any aging would, therefore, have to be done through guest_memfd. Which we don't support and don't want to support :) That we happen to have a matching user space range that maps the guest_memfd is just coincidence from a KVM MMU point of view. > > The invalidate callbacks are also used when userspace memory is marked read-only > for dirty state tracking. I was trying to explaing that, since there is no > backing for the guest_memfd file, host doesn't need to keep track of dirty state > for the memory, and ignoring the invalidate callbacks is correct for all cases. > > I can drop the paragraph entirely, if you think that would make the commit > message clearer. I think the real motivation is: "Mappings in the secondary MMU were established by obtaining folios from guest_memfd directly, not by looking the folios up through the page tables through GUP. Consequently, there is no relationship between the page tables and the secondary MMU: MMU notifiers do not apply." -- Cheers, David