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

[Peter Zijlstra]

On Tue, Jul 28, 2026 at 05:46:30PM +0300, Mike Rapoport wrote:
> 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
> 

The TLBI hack in __kernel_map_pages() makes me wonder how any of this is
correct to begin with. That comment isn't helping.