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

[Mike Rapoport]

On Tue, Jul 28, 2026 at 06:53:56AM -0700, Breno Leitao wrote:
> 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?

Yes:
https://lore.kernel.org/all/20260728-cpa-fixes-v1-0-2ed2352300b3@xxxxxxxxxx

> --breno

-- 
Sincerely yours,
Mike.