Re: [PATCH v9 2/2] mm/memory_hotplug: optimize zone contiguous check when changing pfn range

[Mike Rapoport]

On Tue, Sep 15, 2026 at 05:20:07PM +0200, David Hildenbrand (Arm) wrote:
> On 9/15/26 08:03, Mike Rapoport wrote:
> > Hi,
> > 
> > On Mon, Sep 14, 2026 at 03:29:29AM -0400, Yuan Liu wrote:
> >> When move_pfn_range_to_zone() or remove_pfn_range_from_zone() updates a
> >> zone, set_zone_contiguous() rescans the entire zone pageblock-by-pageblock
> >> to rebuild zone->contiguous. For large zones this is a significant cost
> >> during memory hotplug and hot-unplug.
> >>
> >> diff --git a/mm/mm_init.c b/mm/mm_init.c
> >> index 1533aebafb68..d40a8ff23370 100644
> >> --- a/mm/mm_init.c
> >> +++ b/mm/mm_init.c
> >> @@ -817,22 +817,39 @@ void __meminit init_deferred_page(unsigned long pfn, int nid)
> >>   *   zone/node above the hole except for the trailing pages in the last
> >>   *   section that will be appended to the zone/node below.
> >>   */
> >> -static void __init init_unavailable_range(unsigned long spfn,
> >> -					  unsigned long epfn,
> >> -					  int zone, int node)
> >> +static unsigned long __init init_unavailable_range(unsigned long spfn,
> >> +						   unsigned long epfn,
> >> +						   int zone, int node)
> >>  {
> >> +	unsigned long next_chunk_pfn __maybe_unused = spfn;
> >>  	unsigned long pfn;
> >> -	u64 pgcnt = 0;
> >> +	u64 online_pgcnt = 0, pgcnt = 0;
> >> +	bool is_online = true;
> >>  
> >>  	for_each_valid_pfn(pfn, spfn, epfn) {
> >>  		__init_single_page(pfn_to_page(pfn), pfn, zone, node);
> >>  		__SetPageReserved(pfn_to_page(pfn));
> >>  		pgcnt++;
> >> +
> >> +		/*
> >> +		 * With vmemmap, at this stage all pages in an early section
> >> +		 * have a valid memmap and are marked as online. However, only
> >> +		 * subsections in the subsection map are actually online.
> >> +		 */
> > 
> > I'm having trouble parsing this comment. Shouldn't is say that some
> > subsections in a section can be offline because of holes?
> 
> 
> Maybe extending that a bit we could do
> 
> "With CONFIG_SPARSEMEM_VMEMMAP, early sections have a valid memmap for all PFNs
> and the sections are marked online. However, we might have offline subsections
> in such early sections, indicated by the subsection map. While we must
> initialize the entire valid memmap, account only the online pages according to
> the subsection map as online (see pfn_to_online_page())."
 
init_unavailable_range() already has large comment on top, adding something
like that there makes perfect sense to me :) 

How about a bit different version:

 * The function counts pages that should be added to
 * zone->pages_with_online_memmap.
 * With CONFIG_SPARSEMEM_VMEMMAP there could be offline subsections even though
 * the entire memory map is valid and all the early sections are online.
 * Count only pages in online subsections (see pfn_to_online_page()).
 
> -- 
> Cheers,
> 
> David

-- 
Sincerely yours,
Mike.