Re: [PATCH v4 2/3] xen/mm: allow deferred scrub of physmap populate allocated pages

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

On Wed, Jan 28, 2026 at 03:46:04PM +0100, Jan Beulich wrote:
> On 28.01.2026 13:03, Roger Pau Monne wrote:
> > @@ -275,7 +339,18 @@ static void populate_physmap(struct memop_args *a)
> >              }
> >              else
> >              {
> > -                page = alloc_domheap_pages(d, a->extent_order, a->memflags);
> > +                unsigned int scrub_start = 0;
> > +                nodeid_t node =
> > +                    (a->memflags & MEMF_exact_node) ? MEMF_get_node(a->memflags)
> > +                                                    : NUMA_NO_NODE;
> > +
> > +                page = get_stashed_allocation(d, a->extent_order, node,
> > +                                              &scrub_start);
> > +
> > +                if ( !page )
> > +                    page = alloc_domheap_pages(d, a->extent_order,
> > +                        a->memflags | (d->creation_finished ? 0
> > +                                                            : MEMF_no_scrub));
> 
> I fear there's a more basic issue here that so far we didn't pay attention to:
> alloc_domheap_pages() is what invokes assign_page(), which in turn resets
> ->count_info for each of the pages. This reset includes setting PGC_allocated,
> which ...
> 
> > @@ -286,6 +361,30 @@ static void populate_physmap(struct memop_args *a)
> >                      goto out;
> >                  }
> >  
> > +                if ( !d->creation_finished )
> > +                {
> > +                    unsigned int dirty_cnt = 0;
> > +
> > +                    /* Check if there's anything to scrub. */
> > +                    for ( j = scrub_start; j < (1U << a->extent_order); j++ )
> > +                    {
> > +                        if ( !test_and_clear_bit(_PGC_need_scrub,
> > +                                                 &page[j].count_info) )
> > +                            continue;
> 
> ... means we will now scrub every page in the block, not just those which weren't
> scrubbed yet, and we end up clearing PGC_allocated. All because of PGC_need_scrub
> aliasing PGC_allocated. I wonder how this didn't end up screwing any testing you
> surely will have done. Or maybe I'm completely off here?

Thanks for spotting this!  No, I didn't see any issues.  I don't see
any check for PGC_allocated in free_domheap_pages(), which could
explain the lack of failures?

I will have to allocate with MEMF_no_owner and then do the
assign_pages() call from populate_physmap() after the scrubbing is
done.  Maybe that would work.  Memory allocated using MEMF_no_owner
still consumes the claim pool if a domain parameter is passed to
alloc_heap_pages().

Roger.