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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 17 Dec 2025 10:49:18.6208 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 8ebaa8b8-0c86-43dd-758e-08de3d59edf6 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: DB1PEPF0003922F.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DU0PR08MB7464 Hi Ryan, > On 16/12/2025 16:52, Yeoreum Yun wrote: > > Hi Ryan, > > > >> On 12/12/2025 16:18, Yeoreum Yun wrote: > >>> Some architectures invoke pagetable_alloc() or __get_free_pages() > >>> with preemption disabled. > >>> For example, in arm64, linear_map_split_to_ptes() calls pagetable_alloc() > >>> while spliting block entry to ptes and __kpti_install_ng_mappings() > >>> calls __get_free_pages() to create kpti pagetable. > >>> > >>> Under PREEMPT_RT, calling pagetable_alloc() with > >>> preemption disabled is not allowed, because it may acquire > >>> a spin lock that becomes sleepable on RT, potentially > >>> causing a sleep during page allocation. > >>> > >>> Since above two functions is called as callback of stop_machine() > >>> where its callback is called in preemption disabled, > >>> They could make a potential problem. (sleeping in preemption disabled). > >>> > >>> To address this, introduce pagetable_alloc_nolock() API. > >> > >> I don't really understand what the problem is that you're trying to fix. As I > >> see it, there are 2 call sites in arm64 arch code that are calling into the page > >> allocator from stop_machine() - one via via pagetable_alloc() and another via > >> __get_free_pages(). But both of those calls are passing in GFP_ATOMIC. It was my > >> understanding that the page allocator would ensure it never sleeps when > >> GFP_ATOMIC is passed in, (even for PREEMPT_RT)? > > > > Although GFP_ATOMIC is specify, it only affects of "water mark" of the > > page with __GFP_HIGH. and to get a page, it must grab the lock -- > > zone->lock or pcp_lock in the rmqueue(). > > > > This zone->lock and pcp_lock is spin_lock and it's a sleepable in > > PREEMPT_RT that's why the memory allocation/free using general API > > except nolock() version couldn't be called since > > if "contention" happens they'll sleep while waiting to get the lock. > > > > The reason why "nolock()" can use, it always uses "trylock" with > > ALLOC_TRYLOCK flags. otherwise GFP_ATOMIC also can be sleepable in > > PREEMPT_RT. > > > >> > >> What is the actual symptom you are seeing? > > > > Since the place where called while smp_cpus_done() and there seems no > > contention, there seems no problem. However as I mention in another > > thread > > (https://lore.kernel.org/all/aT%2FdrjN1BkvyAGoi@e129823.arm.com/), > > This gives a the false impression -- > > GFP_ATOMIC are “safe to use in preemption disabled” > > even though they are not in PREEMPT_RT case, I've changed it. > > > >> > >> If the page allocator is somehow ignoring the GFP_ATOMIC request for PREEMPT_RT, > >> then isn't that a bug in the page allocator? I'm not sure why you would change > >> the callsites? Can't you just change the page allocator based on GFP_ATOMIC? > > > > It doesn't ignore the GFP_ATOMIC feature: > > - __GFP_HIGH: use water mark till min reserved > > - __GFP_KSWAPD_RECLAIM: wake up kswapd if reclaim required. > > > > But, it's a restriction -- "page allocation / free" API cannot be called > > in preempt-disabled context at PREEMPT_RT. > > > > That's why I think it's wrong usage not a page allocator bug. > > I've taken a look at this and I agree with your analysis. Thanks for explaining. > > Looking at other stop_machine() callbacks, there are some that call printk() and > I would assume that spinlocks could be taken there which may present the same > kind of issue or PREEMPT_RT? (I'm guessing). I don't see any others that attempt > to allocate memory though. IIRC, there was a problem related for printk while try to grab pl011_console related lock (spin_lock) while holding console_lock(raw_spin_lock) in v6.10.0-rc7 at rpi5: [ 230.381263] CPU: 2 PID: 5574 Comm: syz.4.1695 Not tainted 6.10.0-rc7-01903-g52828ea60dfd #3 [ 230.381479] Hardware name: linux,dummy-virt (DT) [ 230.381565] Call trace: [ 230.381607] dump_backtrace+0x318/0x348 [ 230.381727] show_stack+0x4c/0x80 [ 230.381875] dump_stack_lvl+0x214/0x328 [ 230.382159] dump_stack+0x3c/0x58 [ 230.382456] __lock_acquire+0x4398/0x4720 [ 230.382683] lock_acquire+0x648/0xb70 [ 230.382928] _raw_spin_lock_irqsave+0x138/0x240 [ 230.383121] pl011_console_write+0x240/0x8a0 [ 230.383356] console_flush_all+0x708/0x1368 [ 230.383571] console_unlock+0x180/0x440 [ 230.383742] vprintk_emit+0x1f8/0x9d0 [ 230.383832] vprintk_default+0x64/0x90 [ 230.383914] vprintk+0x2d0/0x400 [ 230.383971] _printk+0xdc/0x128 [ 230.384229] hrtimer_interrupt+0x8f0/0x920 [ 230.384414] arch_timer_handler_virt+0xc0/0x100 [ 230.384812] handle_percpu_devid_irq+0x20c/0x4e0 [ 230.385053] generic_handle_domain_irq+0xc0/0x120 [ 230.385367] gic_handle_irq+0x88/0x360 [ 230.385559] call_on_irq_stack+0x24/0x70 [ 230.385801] do_interrupt_handler+0xf8/0x200 [ 230.386092] el1_interrupt+0x68/0xc0 [ 230.386434] el1h_64_irq_handler+0x18/0x28 [ 230.386716] el1h_64_irq+0x64/0x68 [ 230.386853] __sanitizer_cov_trace_const_cmp2+0x30/0x68 [ 230.387026] alloc_pages_mpol_noprof+0x170/0x698 [ 230.387309] vma_alloc_folio_noprof+0x128/0x2a8 [ 230.387610] vma_alloc_zeroed_movable_folio+0xa0/0xe0 [ 230.387822] folio_prealloc+0x5c/0x280 [ 230.388008] do_wp_page+0xc30/0x3bc0 [ 230.388206] __handle_mm_fault+0xdb8/0x2ba0 [ 230.388448] handle_mm_fault+0x194/0x8a8 [ 230.388676] do_page_fault+0x6bc/0x1030 [ 230.388924] do_mem_abort+0x8c/0x240 [ 230.389056] el0_da+0xf0/0x3f8 [ 230.389178] el0t_64_sync_handler+0xb4/0x130 [ 230.389452] el0t_64_sync+0x190/0x198 But this problem is gone when I try with some of patches in rt-tree related for printk which are merged in current tree (https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/log/?h=linux-6.10.y-rt-rebase). So I think printk() wouldn't be a problem. > > Anyway, to fix the 2 arm64 callsites, I see 2 possible approaches: > > - Call the nolock variant (as you are doing). But that would just convert a > deadlock to a panic; if the lock is held when stop_machine() runs, without your > change, we now have a deadlock due to waiting on the lock inside stop_machine(). > With your change, we notice the lock is already taken and panic. I guess it is > marginally better, but not by much. Certainly I would just _always_ call the > nolock variant regardless of PREEMPT_RT if we take this route; For !PREEMPT_RT, > the lock is guarranteed to be free so nolock will always succeed. > > - Preallocate the memory before entering stop_machine(). I think this would be > much more robust. For kpti_install_ng_mappings() I think you could hoist the > allocation/free out of stop_machine() and pass the pointer in pretty easily. For > linear_map_split_to_ptes() its a bit more complex; Perhaps, we need to walk the > pgtable to figure out how much to preallocate, allocate it, then set it up as a > special allocator, wrapped by an allocation function and modify the callchain to > take a callback function instead of gfp flags. > > What do you think? Definitely, second suggestoin is much better. My question is whether *memory contention* really happen in the point both functions are called. Above two functions are called as last step of "smp_init()" -- smp_cpus_done(). If we can be sure, I think we don't need to go to complex way and I believe the reason why we couldn't find out this problem, even using GFP_ATOMIC in PREEMPT_RT since there was *no contection* in this time of both functions are called. That's why I first try with the "simple way". What do you think? -- Sincerely, Yeoreum Yun