Re: [PATCH 2/2] xen/mm: do not assign pages to a domain until they are scrubbed

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Mar 25, 2026 at 05:26:01PM +0100, Roger Pau Monné wrote:
> On Wed, Mar 25, 2026 at 03:56:05PM +0100, Jan Beulich wrote:
> > On 25.03.2026 11:08, Roger Pau Monne wrote:
> > >  * Disallow XENMEM_decrease_reservation until the domain has finished
> > >    creation would fix the issue of pages being freed while pending scrub,
> > >    but it's not clear there might be other usages that would be problematic,
> > >    as get_page() on non-scrubbed pages would still return success.
> > 
> > I agree this is of concern.
> > 
> > > --- a/xen/common/memory.c
> > > +++ b/xen/common/memory.c
> > > @@ -388,6 +388,12 @@ static void populate_physmap(struct memop_args *a)
> > >                              goto out;
> > >                          }
> > >                      }
> > > +
> > > +                    if ( assign_page(page, a->extent_order, d, memflags) )
> > > +                    {
> > > +                        free_domheap_pages(page, a->extent_order);
> > 
> > The pages don't have an owner set yet, so that function will go straight
> > to free_heap_pages(), needlessly passing "true" as last argument. Correct,
> > but (for large pages, which the stashing is about) highly inefficient.
> 
> My bad, I was sure I was using the same freeing function as
> alloc_domheap_pages() on failure to assign, but I clearly wasn't.  I
> will switch to using free_heap_pages().

Coming back to this, I can export free_heap_pages(), but then the call
would also unconditionally have need_scrub == true, as the pages have
been allocated without scrubbing.  So I don't see much benefit of
using free_heap_pages(), the more that it requires making such
function global then.

Thanks, Roger.