[PATCH v3] mm: don't schedule deferred kernel page table freeing while booting

[Mikhail Gavrilov]

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]  <TASK>
  [   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) <david@xxxxxxxxxx>
Suggested-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
Link: https://lore.kernel.org/20260924064321.23787-1-mikhail.v.gavrilov@xxxxxxxxx
---
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@xxxxxxxxx
v1: https://lore.kernel.org/20260924064321.23787-1-mikhail.v.gavrilov@xxxxxxxxx

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