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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 25 Jul 2025 04:24:39.7631 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 39a7838f-6be2-42d0-f9a4-08ddcb332c00 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: AMS0EPF000001A3.eurprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: GV1PR08MB10453 On 24/07/25 5:28 pm, Catalin Marinas wrote: > On Thu, Jul 24, 2025 at 04:10:18PM +0530, Dev Jain wrote: >> On 24/07/25 1:49 pm, Catalin Marinas wrote: >>> On Thu, Jul 03, 2025 at 08:44:41PM +0530, Dev Jain wrote: >>>> arm64 currently changes permissions on vmalloc objects locklessly, via >>>> apply_to_page_range, whose limitation is to deny changing permissions for >>>> block mappings. Therefore, we move away to use the generic pagewalk API, >>>> thus paving the path for enabling huge mappings by default on kernel space >>>> mappings, thus leading to more efficient TLB usage. However, the API >>>> currently enforces the init_mm.mmap_lock to be held. To avoid the >>>> unnecessary bottleneck of the mmap_lock for our usecase, this patch >>>> extends this generic API to be used locklessly, so as to retain the >>>> existing behaviour for changing permissions. >>> Is it really a significant bottleneck if we take the lock? I suspect if >>> we want to make this generic and allow splitting, we'll need a lock >>> anyway (though maybe for shorter intervals if we only split the edges). >> The splitting primitive may or may not require a lock, Ryan and Yang had >> some discussion on the linear map block mapping thread. I am assuming >> that since we didn't need a lock in the past, there is no need to take it now, >> since we are only changing the pagetable walk API being used. > I vaguely remember Ryan's and Yang's discussion. I'd have to revisit it. > In the past we were not replacing block/table entries since there was no > page table splitting. If we are to add some splitting, even at the > edges, what would prevent some other caller of this API overlapping and > attempting to do the same split? It's not just vmalloc ranges but the > linear map as well that's touched by __change_memory_common(). Sorry I wasn't clear enough, what I meant to say was that the locking will be the concernment of the splitting primitive - once that is done, update_range_prot() does not need to take the lock. >