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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jan 29, 2026 at 08:53:05AM +0100, Jan Beulich wrote:
> On 28.01.2026 20:06, Roger Pau Monné wrote:
> > 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?
> 
> Maybe. PGC_allocated consumes a page ref, so I would have expected accounting
> issues.
> 
> > 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().
> 
> Technically this looks like it might work, but it's feeling as if this was
> getting increasingly fragile. I'm also not quite sure whether MEMF_no_owner
> allocations should consume claimed pages. Imo there are arguments both in
> favor and against such behavior.
> 
> We may want to explore the alternative of un-aliasing the two PGC_*.

I expected the PGC_ bits would be all consumed, but I see there's a
range that are still empty, so it might indeed be easier to remove the
alias.  Let me give that a try.

Thanks, Roger.