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

[Liu, Yuan1]

> -----Original Message-----
> From: Mike Rapoport <rppt@xxxxxxxxxx>
> Sent: Wednesday, September 16, 2026 2:58 AM
> To: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Cc: Liu, Yuan1 <yuan1.liu@xxxxxxxxx>; Oscar Salvador <osalvador@xxxxxxx>;
> Wei Yang <richard.weiyang@xxxxxxxxx>; linux-mm@xxxxxxxxx; Zou, Nanhai
> <nanhai.zou@xxxxxxxxx>; Chen Zhang <zhangchen.kidd@xxxxxx>; Zeng, Jason
> <jason.zeng@xxxxxxxxx>; Chen, Yu C <yu.c.chen@xxxxxxxxx>; Deng, Pan
> <pan.deng@xxxxxxxxx>; Li, Tianyou <tianyou.li@xxxxxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v9 2/2] mm/memory_hotplug: optimize zone contiguous
> check when changing pfn range
> 
> 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()).

Hi David

Should I prepare and send a new version based on Mike's latest suggested changes?

Best Regards,
Liu, Yuan

> > --
> > Cheers,
> >
> > David
> 
> --
> Sincerely yours,
> Mike.