Re: [PATCH v5 09/16] kexec: enable KHO support for memory preservation

[Mike Rapoport <rppt@kernel.org>]

On Mon, Mar 24, 2025 at 05:07:36PM -0300, Jason Gunthorpe wrote:
> On Mon, Mar 24, 2025 at 02:18:34PM -0400, Mike Rapoport wrote:
> > On Sun, Mar 23, 2025 at 03:55:52PM -0300, Jason Gunthorpe wrote:
> > > On Sat, Mar 22, 2025 at 03:12:26PM -0400, Mike Rapoport wrote:
> > >  
> > > > > > +		page->private = order;
> > > > > 
> > > > > Can't just set the page order directly? Why use private?
> > > > 
> > > > Setting the order means recreating the folio the way prep_compound_page()
> > > > does. I think it's better to postpone it until the folio is requested. This
> > > > way it might run after SMP is enabled. 
> > > 
> > > I see, that makes sense, but also it could stil use page->order..
> > 
> > But there's no page->order :)
> 
> I mean this:
> 
> static inline unsigned int folio_order(const struct folio *folio)
> {
>         if (!folio_test_large(folio))
>                 return 0;
>         return folio->_flags_1 & 0xff;
> }

I don't think it's better than page->private, KHO will need to
prep_compound_page() anyway so these will be overwritten there.
And I don't remember, but having those set before prep_compound_page()
might trigger VM_BUG_ON_PGFLAGS().
  
> Jason

-- 
Sincerely yours,
Mike.