Re: [PATCH 1/5] x86/mm/pat: introcude cpa_lock() and cpa_unlock()

On Tue, Jul 28, 2026 at 04:31:35PM +0200, Peter Zijlstra wrote:
> On Tue, Jul 28, 2026 at 07:30:27AM -0700, Dave Hansen wrote:
> > On 7/28/26 07:21, Peter Zijlstra wrote:
> > > There was already a patch merged that removed the shole debug_pagealloc
> > > exception. Is that not better?
> > 
> > As I'm scanning through email this morning, there's another issue that
> > popped up with that patch. It's causing hangs on boot.
> > 
> > It's looking like debug pagealloc not taking the lock is actually
> > functional, not an optimization. Although, I hesitate to say
> > "functional" and would prefer to use much less nice words to describe it.
> 
> Yeah, lets figure out why that is before we retain this wart ;-)

As Lorenzo said:

  __kernel_map_pages() can be called from irq context:

  < GFP_ATOMIC context >
  kfree() or whatever
  -> ...
  -> __free_pages_prepare()
  -> debug_pagealloc_unmap_pages()
  -> __kernel_map_pages()
  -> __change_page_attr_set_clr()
  -> cpa_lock


-- 
Sincerely yours,
Mike.