Re: [PATCH 2/2] xen/mm: limit non-scrubbed allocations to a specific order

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

On Wed, Jan 14, 2026 at 09:48:59AM +0100, Jan Beulich wrote:
> On 13.01.2026 15:01, Roger Pau Monné wrote:
> > On Fri, Jan 09, 2026 at 12:19:26PM +0100, Jan Beulich wrote:
> >> On 08.01.2026 18:55, Roger Pau Monne wrote:
> >>> --- a/xen/common/memory.c
> >>> +++ b/xen/common/memory.c
> >>> @@ -279,6 +279,18 @@ static void populate_physmap(struct memop_args *a)
> >>>  
> >>>                  if ( unlikely(!page) )
> >>>                  {
> >>> +                    nodeid_t node = MEMF_get_node(a->memflags);
> >>> +
> >>> +                    if ( memory_scrub_pending(node) ||
> >>> +                         (node != NUMA_NO_NODE &&
> >>> +                          !(a->memflags & MEMF_exact_node) &&
> >>> +                          memory_scrub_pending(node = NUMA_NO_NODE)) )
> >>> +                    {
> >>> +                        scrub_free_pages(node);
> >>> +                        a->preempted = 1;
> >>> +                        goto out;
> >>> +                    }
> >>
> >> At least for order 0 requests there's no point in trying this. With the
> >> current logic, actually for orders up to MAX_DIRTY_ORDER.
> > 
> > Yes, otherwise we might force the CPU to do some scrubbing work when
> > it won't satisfy it's allocation request anyway.
> > 
> >> Further, from a general interface perspective, wouldn't we need to do the
> >> same for at least XENMEM_increase_reservation?
> > 
> > Possibly yes.  TBH I would also be fine with strictly limiting
> > XENMEM_increase_reservation to 2M order extents, even for the control
> > domain.  The physmap population is the only that actually requires
> > bigger extents.
> 
> Hmm, that's an option, yes, but an ABI-changing one.

I don't think it changes the ABI: Xen has always reserved the right to
block high order allocations.  See for example how max_order() has
different limits depending on the domain permissions, and I would not
consider those limits part of the ABI, they can be changed from the
command line.

Thanks, Roger.