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

[Mike Rapoport <rppt@kernel.org>]

On Tue, Sep 15, 2026 at 10:40:01AM +0000, Liu, Yuan1 wrote:
> > -----Original Message-----
> > From: Mike Rapoport <rppt@kernel.org>
> > Sent: Tuesday, September 15, 2026 2:04 PM
> > To: Liu, Yuan1 <yuan1.liu@intel.com>
> > Cc: David Hildenbrand <david@kernel.org>; Oscar Salvador
> > <osalvador@suse.de>; Wei Yang <richard.weiyang@gmail.com>; linux-
> > mm@kvack.org; Zou, Nanhai <nanhai.zou@intel.com>; Chen Zhang
> > <zhangchen.kidd@jd.com>; Zeng, Jason <jason.zeng@intel.com>; Chen, Yu C
> > <yu.c.chen@intel.com>; Deng, Pan <pan.deng@intel.com>; Li, Tianyou
> > <tianyou.li@intel.com>; linux-kernel@vger.kernel.org
> > Subject: Re: [PATCH v9 2/2] mm/memory_hotplug: optimize zone contiguous
> > check when changing pfn range
> > 
> > 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?
> > 
> > Other than that
> > 
> > Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> 
> Maybe we could add some more comments here
> /*
>  * With vmemmap, at this stage all pages in an early section
>  * have a valid memmap and are marked as online. However,
>  * subsection-sized holes within a section are offline, only
>  * subsections in the subsection map are actually online.
>  */

I don't see how adding almost identical cryptic comment in another place
helps.
 
> > > +#ifdef CONFIG_SPARSEMEM_VMEMMAP
> > > +		if (pfn >= next_chunk_pfn) {
> > > +			is_online = pfn_section_valid(__pfn_to_section(pfn),
> > pfn);
> > > +			next_chunk_pfn = min(SUBSECTION_ALIGN_UP(pfn + 1),
> > epfn);
> > > +		}
> > > +#endif
> > > +		if (is_online)
> > > +			online_pgcnt++;
> > >  	}
> > >
> > >  	if (pgcnt)
> > >  		pr_info("On node %d, zone %s: %lld pages in unavailable
> > ranges\n",
> > >  			node, zone_names[zone], pgcnt);
> > > +	return online_pgcnt;
> > >  }
> > >
> > >  /*
> > 
> > --
> > Sincerely yours,
> > Mike.

-- 
Sincerely yours,
Mike.