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

On Tue, Jul 28, 2026 at 04:55:28PM +0200, Peter Zijlstra wrote:
> 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.
>

I think there's probably more horror shows there, it's a case of put out the
most immediate fires, leave the probably remaining flaming trash pile for later
;)

I thought x86 allowing arbitrary mm ptdump walks was crazy enough but this debug
pagealloc stuff kinda pushes things further crazy-wise...