Booting with a boot-time function tracer and a filter, for example ftrace=function ftrace_filter=pud_free_pmd_page panics on 7.3-rc4 as soon as the tracer starts: [ 23.531178] Starting tracer 'function' [ 23.675800] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000038: 0000 [#1] SMP KASAN NOPTI [ 23.819917] KASAN: null-ptr-deref in range [0x00000000000001c0-0x00000000000001c7] [ 23.964025] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 7.3.0-rc4-fe2ec83746e5-with-fixes-v2+ #195 PREEMPT(undef) [ 24.252248] RIP: 0010:__queue_work+0xab/0xf00 [ 25.981629] Call Trace: [ 26.125727] [ 26.413912] ? pagetable_free_kernel+0x20/0x120 [ 26.990283] queue_work_on+0x97/0xf0 [ 27.134382] __cpa_collapse_large_pages+0x501/0x6f0 [ 27.566662] cpa_flush+0x394/0x620 [ 27.998953] change_page_attr_set_clr+0x321/0x4a0 [ 29.151729] set_memory_rox+0xa2/0xf0 [ 29.584018] create_trampoline+0x431/0x6f0 ... [ 44.343347] Kernel panic - not syncing: Attempted to kill the idle task! The boot-time tracer is started from early_trace_init(), which runs before workqueue_init_early(). Making its trampoline read-only splits a large page, and CPA collapses it again right away. The split table has been a kernel page table since commit 9e4a3ec3411b ("x86/mm/pat: Allocate split page tables as kernel page tables"), so the collapse frees it through pagetable_free_kernel(), which queues work on system_percpu_wq - still NULL at that point. That commit is correct in itself; it only lets CPA reach pagetable_free_kernel() before the workqueue that function relies on exists. Keep putting the table on the list, but don't schedule the work while the system is still booting. A core_initcall schedules it once to free whatever was queued by then. Fixes: 9e4a3ec3411b ("x86/mm/pat: Allocate split page tables as kernel page tables") Suggested-by: David Hildenbrand (Arm) Suggested-by: Lorenzo Stoakes (ARM) Cc: stable@vger.kernel.org Signed-off-by: Mikhail Gavrilov Link: https://lore.kernel.org/20260924064321.23787-1-mikhail.v.gavrilov@gmail.com --- v3: - Schedule the work once from a core_initcall to free whatever was queued during boot (Lorenzo Stoakes, Dave Hansen, David Hildenbrand). late_initcall would work just as well; workqueues exist from workqueue_init() on. - Keep the fix in pagetable_free_kernel() rather than skipping the collapse during boot (Mike Rapoport): that would only avoid this caller, and any other early free would still need a workqueue. v2: https://lore.kernel.org/20260924092307.22813-1-mikhail.v.gavrilov@gmail.com v1: https://lore.kernel.org/20260924064321.23787-1-mikhail.v.gavrilov@gmail.com Tested on a Ryzen 9 7950X with a Radeon RX 7900 XTX (lockdep, KASAN), 7.3-rc4 plus unrelated local changes, by booting with ftrace=function ftrace_filter=pud_free_pmd_page,pagetable_free_kernel,kernel_pgtable_work_func,kernel_pgtable_drain_early The boot that panicked without the fix completes, and the trace shows kernel_pgtable_drain_early() and then kernel_pgtable_work_func() before any other kernel page table is freed. mm/pgtable-generic.c | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-) diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c index b91b1a98029c..cd227fc05d2d 100644 --- a/mm/pgtable-generic.c +++ b/mm/pgtable-generic.c @@ -438,12 +438,30 @@ static void kernel_pgtable_work_func(struct work_struct *work) __pagetable_free(pt); } +static void schedule_kernel_pgtable_free(void) +{ + schedule_work(&kernel_pgtable_work.work); +} + void pagetable_free_kernel(struct ptdesc *pt) { spin_lock(&kernel_pgtable_work.lock); list_add(&pt->pt_list, &kernel_pgtable_work.list); spin_unlock(&kernel_pgtable_work.lock); - schedule_work(&kernel_pgtable_work.work); + /* + * The workqueue may not exist yet while the system is booting. + * kernel_pgtable_drain_early() schedules the work once it does. + */ + if (system_state != SYSTEM_BOOTING) + schedule_kernel_pgtable_free(); +} + +static int __init kernel_pgtable_drain_early(void) +{ + /* Free the kernel page tables queued while booting. */ + schedule_kernel_pgtable_free(); + return 0; } +core_initcall(kernel_pgtable_drain_early); #endif -- 2.55.0