RE: [PATCH v6 1/2] mm/memory_hotplug: optimize zone contiguous check when changing pfn range

[Liu, Yuan1]

> -----Original Message-----
> From: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Sent: Friday, August 7, 2026 7:30 PM
> To: Liu, Yuan1 <yuan1.liu@xxxxxxxxx>; Oscar Salvador <osalvador@xxxxxxx>;
> Mike Rapoport <rppt@xxxxxxxxxx>; Wei Yang <richard.weiyang@xxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx; Zou, Nanhai <nanhai.zou@xxxxxxxxx>; Deng, Pan
> <pan.deng@xxxxxxxxx>; Li, Tianyou <tianyou.li@xxxxxxxxx>; Chen Zhang
> <zhangchen.kidd@xxxxxx>; Zeng, Jason <jason.zeng@xxxxxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v6 1/2] mm/memory_hotplug: optimize zone contiguous
> check when changing pfn range
> 
> On 8/6/26 11:52, Liu, Yuan1 wrote:
> >> -----Original Message-----
> >> From: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> >> Sent: Thursday, August 6, 2026 4:46 PM
> >> To: Liu, Yuan1 <yuan1.liu@xxxxxxxxx>; Oscar Salvador
> <osalvador@xxxxxxx>;
> >> Mike Rapoport <rppt@xxxxxxxxxx>; Wei Yang <richard.weiyang@xxxxxxxxx>
> >> Cc: linux-mm@xxxxxxxxx; Zou, Nanhai <nanhai.zou@xxxxxxxxx>; Deng, Pan
> >> <pan.deng@xxxxxxxxx>; Li, Tianyou <tianyou.li@xxxxxxxxx>; Chen Zhang
> >> <zhangchen.kidd@xxxxxx>; Zeng, Jason <jason.zeng@xxxxxxxxx>; linux-
> >> kernel@xxxxxxxxxxxxxxx
> >> Subject: Re: [PATCH v6 1/2] mm/memory_hotplug: optimize zone contiguous
> >> check when changing pfn range
> >>
> >>
> >>>
> >>> Will do.
> >>>
> >>>
> >>> pages_with_online_memmap counts all PFNs where pfn_to_online_page() is
> >>> valid. With CONFIG_SPARSEMEM_VMEMMAP, pfn_section_valid() operates at
> >>> PAGES_PER_SUBSECTION granularity — when any page in a subsection has
> >>> memory, the entire subsection is valid/online. So we align to
> subsection
> >>> boundaries to include hole pages within partially-populated
> subsections.
> >>
> >> But we must never account exceeding the zone range. So I don't
> understand
> >> why we
> >> would have to care about PAGES_PER_SUBSECTION here at all?
> >
> > We never account beyond the zone range, because `sub_start` and
> > `sub_end` are still clamped to the zone boundaries after the
> > alignment.
> >
> > The subsection alignment is needed for hole PFNs between memblocks
> > within the zone. These hole PFNs are valid for
> > `pfn_to_online_page()`, because they sit in a subsection that has
> > memory, so the whole subsection's memmap is online.
> >
> > |----------- zone range -----------|
> >
> > +-----------+---------+------------+
> > + memblock 1|  hole   | memblock 2 |
> > +-----------+---------+------------+
> >                ^
> >                |
> >                |
> >      subsection boundary
> 
> Right, but init_unavailable_range() can just return how many were actually
> initalized? Why can't we piggy-back on that?
> 
> I think we had something similar previously, why can't we use that?
> 
> We know the zone span, so we can just account all the mmap in the zone
> span
> that we initialize.
> 
> What is the problem with that?

Hi David

My understanding is that init_unavailable_range() initializes all
PFNs that satisfy pfn_valid(), but not all of them satisfy
pfn_to_online_page(), since some PFNs belong to subsections that are
not online.

You previously mentioned:

  pfn_valid() says early sections always have a full memmap, so even invalid
  subsections have a memmap. pfn_to_online_page() says an invalid subsection
  cannot be online and its content must be stale. for_each_valid_pfn() follows
  pfn_valid() semantics, and we use it to initialize memmap that is not going
  to be online and account it as pages_with_online_memmap, which is wrong.

  The cleanest approach is to avoid allocating memmap for subsections, which
  also removes the special early-section handling from pfn_valid() and
  for_each_valid_pfn().

I also share the concerns raised by Sashiko in the analysis below [1]:

  Scanners like isolate_migratepages_block() will then blindly iterate through
  the pageblock and access the completely uninitialized struct pages of the hole,
  leading to functional errors or kernel panics when reading these zero-filled
  structures via macros like PageHuge() or page_zone().

That's why we went with the current approach in v6 instead of your
earlier suggestion. I'd really appreciate your guidance on which
direction you think would be more appropriate.

[1] https://sashiko.dev/#/patchset/20260520093457.3719960-1-yuan1.liu%40intel.com

Best Regards,
Liu, Yuan

> Cheers,
> 
> David