Re: [PATCH] x86/mm/pat: don't gate cpa_lock on debug_pagealloc_enabled()

[Breno Leitao <leitao@debian.org>]

On Wed, Jul 15, 2026 at 05:45:19PM +0300, Mike Rapoport wrote:
> @@ -1235,11 +1234,9 @@ static int split_large_page(struct cpa_data *cpa, pte_t *kpte,
>  {
>  	struct ptdesc *ptdesc;
>  
> -	if (!debug_pagealloc_enabled())
> -		spin_unlock(&cpa_lock);
> +	spin_unlock(&cpa_lock);
>  	ptdesc = pagetable_alloc(GFP_KERNEL, 0);
> -	if (!debug_pagealloc_enabled())
> -		spin_lock(&cpa_lock);
> +	spin_lock(&cpa_lock);

A DEBUG_PAGEALLOC + KASAN kernel from linux-next-20260727 fails to finish
booting on a large VM (many CPUs + large memory).  Boot stalls in early
userspace with a non-responsive CSD lock; it never reaches a login.

I bisected it to this patch, which is the following on linux-next:

  5fce67641a3e ("x86/mm/pat: Don't gate cpa_lock on debug_pagealloc_enabled()")

Reverting it (restoring the !debug_pagealloc_enabled() gating around
cpa_lock) makes the same kernel boot reliably.

Symptom
-------

Boot hangs during udev coldplug (concurrent module loading, i.e. concurrent
set_memory()).  With csdlock_debug=1 the console shows:

  smp: csd: Detected non-responsive CSD lock (#1) on CPU#55, waiting
       5000000019 ns for CPU#57 do_flush_tlb_all+0x0/0x30(0x0).
  smp:  csd: CSD lock (#1) unresponsive.
  Sending NMI from CPU 55 to CPUs 57:
  NMI backtrace for cpu 57
  CPU: 57 UID: 0 PID: 860 Comm: (udev-worker) Tainted: G      E   \
       7.2.0-rc5-next-20260727 #1 PREEMPT(full)
  RIP: 0010:queued_spin_lock_slowpath+0xae/0xd30
  Call Trace:
   <TASK>
   do_raw_spin_lock+0x268/0x2f0
   __change_page_attr_set_clr+0x32d/0x21f0
   ...

CPU#55 is stuck in flush_tlb_all() waiting for CPU#57 to answer the IPI;
CPU#57 is spinning on cpa_lock (RBX resolves to cpa_lock) inside
__change_page_attr_set_clr().  The clocksource watchdog also reports
remote CPUs timing out on the same IPIs just before this.

Is this a known issue?
--breno