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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 18 Dec 2025 15:55:45.6139 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: b7571980-a0fd-4aec-22d1-08de3e4de7e5 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: DU6PEPF0000A7DD.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DU0PR08MB7416 > On 18/12/2025 15:01, Yeoreum Yun wrote: > > Hi Ryan, > > > >> On 17/12/2025 18:20, Yeoreum Yun wrote: > >>> Current linear_map_split_to_ptes() allocate the pagetable > >>> while split linear mapping to ptes by stop_machine() with GFP_ATOMIC. > >>> > >>> This is fine for non-PREEMPR_RT case. > >>> However It's a problem in PREEMPR_RT case since > >>> generic memory allocation/free APIs (e.x) pgtable_alloc(), __get_free_pages and etc > >>> couldn't be called in non-preemptible context except _nolock() APIs > >>> since generic memory allocation/free APIs are *sleepable* for using *spin_lock()* > >>> > >>> IOW, calling a pgtable_alloc() even with GFP_ATOMIC > >>> doesn't allow in __linear_map_split_to_pte() executed by stopper thread > >>> where preemption is disabled in PREEMPR_RT. > >>> > >>> To address this, divide linear_map_maybe_split_ptes: > >>> - collect number of pages to require for spliting. > >>> - allocate the required number of pages for spliting. > >>> - with pre-allocate page, split the linear map. > >> > >> Thanks for working on this fix! > >> > >> First some high level comments: I'm not a huge fan of the approach with the > >> different modes to modify behaviour. > >> > >> For the first step of figuring out the number of pages required; there is no > >> need to be inside stop_machine() for that. Can should be able to just walk the > >> linear map without any locking since it is created once and remains static, with > >> the exception of hotplug, but that is not enabled yet. > >> > >> I think it would be cleaner to just create separate walker callbacks rather than > >> repurpose the existing walker. > > > > Okay. I'll repsin with separate walker. > > > >> > >> Then you could simply change the gfp flag to a callback function pointer and > >> pass the desired allocator as a function pointer. This would match the existing > >> patterns used in mmu.c today. > >> > >> The preallocated page list could be stored in a global static variable and you > >> could create an allocation function that just strims a page from that list. See > >> kpti_ng_pgd_alloc() for an existing example. That assumes a contiguous block, > >> but you could generalize it to hold a list of struct pages and reuse it for both > >> cases? > > > > If we use walker callbacks, I think it not only pass the callback > > function, but it need to pass the gfp flag too since split_xxx() is > > called by another functions. (i.e) set_memory_xxx() > > > > I think I can remove the mode with the suggestion of @David > > but, I think it would be still use split_args with information of > > gfp and preallocate buffer. > > I don't think that's necessary; all other users pass GFP_PGTABLE_KERNEL, so we > just need one allocator function that allocates with GFP_PGTABLE_KERNEL and > another allocator function that allocates from the prealloc'ed list. Then the > user passes the function they want to use. > > That would work, I think? True. I forgot I removed GFP_ATOMIC myself for pgtable alloc. Okay. then I'll move the preallocated buffer information to global and just pass function pointer then. Thanks -- Sincerely, Yeoreum Yun