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

[Lorenzo Stoakes (ARM)]

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

Yup :)

Without the debug stuff you have this issue.

With it you don't...

Also without it Denis's patch is broken.

This series Mike's providing is fairly exhuastively the best way to fix the
various issues, grouped together and parted from mm to make everybody's life a
little easier...

>
>
> --
> Sincerely yours,
> Mike.

Cheers, Lorenzo