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

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

Jan