RE: [PATCH v6 0/2] mm/memory_hotplug: optimize zone contiguous check when changing pfn range

Hi David,

The reworked implementation updates pages_with_online_memmap in
memmap_init_zone_range(), counting only PFNs that satisfy
pfn_to_online_page(). This eliminates the extra PFN range walk used
in the previous version.

Does this look like the right direction, or am I missing something?
I'd appreciate your thoughts.

Thanks!

Best Regards,
Liu, Yuan1

> -----Original Message-----
> From: Liu, Yuan1 <[email protected]>
> Sent: Thursday, July 23, 2026 4:50 PM
> To: David Hildenbrand <[email protected]>; Oscar Salvador
> <[email protected]>; Mike Rapoport <[email protected]>; Wei Yang
> <[email protected]>
> Cc: [email protected]; Zou, Nanhai <[email protected]>; Liu, Yuan1
> <[email protected]>; Deng, Pan <[email protected]>; Li, Tianyou
> <[email protected]>; Chen Zhang <[email protected]>; Zeng, Jason
> <[email protected]>; [email protected]
> Subject: [PATCH v6 0/2] mm/memory_hotplug: optimize zone contiguous check
> when changing pfn range
> 
> This series introduces a pages_with_online_memmap member into struct
> zone to avoid pageblock-by-pageblock scans across the entire zone and
> improve memory hotplug performance.
> 
> Reworked how pages_with_online_memmap accounts for PFNs in early
> boot memory in response to Sashiko's review comments [2], which
> identified potential issues with the previous implementation.
> 
> Approach
> ========
> Add a new zone member pages_with_online_memmap that tracks the number of
> pages within the zone span that have an online memory map (including
> present pages and memory holes smaller than a subsection whose memory
> map has been initialized). When spanned_pages == pages_with_online_memmap
> the zone is contiguous and pfn_to_page() can be called on any PFN in the
> zone span without further pfn_valid() checks.
> 
> For early boot memory, pages_with_online_memmap is calculated in
> memmap_init_zone_range(). Since every PFN within a memblock region
> satisfies pfn_to_online_page(), the calculation aligns both boundaries
> of each memblock range to PAGES_PER_SUBSECTION and counts the aligned
> pages that fall within the zone span. If two adjacent memblock regions
> share a subsection (i.e. the hole between them is smaller than a
> subsection), the overlapping subsection pages are counted only once.
> For hotplugged memory, pages_with_online_memmap is updated through
> adjust_present_page_count(), which is called during memory online and
> offline.
> 
> Only pages that fall within the current zone span are accounted towards
> pages_with_online_memmap. A "too small" value is safe, it merely
> prevents detecting a contiguous zone.
> 
> The contiguity check using pages_with_online_memmap is stricter than the
> old pageblock-by-pageblock scan. The old set_zone_contiguous() iterated
> at pageblock granularity via pageblock_pfn_to_page(), so a zone could be
> marked contiguous even if a subsection-sized hole existed within a
> pageblock. The new check requires spanned_pages ==
> pages_with_online_memmap,
> meaning every PFN in the zone span must satisfy pfn_to_online_page().
> 
> Performance
> ===========
> 1. For VM hotplug performance data, please refer to Patch 1.
> 2. This series also benefits CXL hotplug. Performance results are
>    as follows
>    https://lore.kernel.org/all/20260409023552.GA2807@AE/
> 
> Tested cases
> ============
> 1. Hotplug/unplug correctness with both online_movable and online
>    policies, including partial unplug, middle-block offline gaps, and
>    edge-block span shrink.
> 2. Large scale (256G/512G) plug/unplug performance.
> 3. Boot-time subsection holes (aligned/unaligned, in-zone and
>    cross-zone) with correct pages_with_online_memmap accounting.
> 4. kernelcore=mirror: verified no overcounting.
> 
> Patch overview
> ==============
> This series is based on the kernelcore-mirror branch [1].
> 
> Patch 1 introduces pages_with_online_memmap to replace
> pageblock-by-pageblock scans across the entire zone for zone contiguity
> checks.
> 
> Patch 2 avoids incorrectly shrinking the zone span during memory unplug
> when a memory or hole boundary falls in the middle of a subsection.
> 
> v6 changes:
>   1. Dropped patches 1-3 from v5. The kernelcore=mirror overlap issue
>      that caused pages_with_online_memmap to be overcounted has been
>      resolved by the series [1].
>   2. Reworked pages_with_online_memmap accounting during early boot.
> 
> v5:
>     https://lore.kernel.org/linux-mm/20260520093457.3719960-1-
> [email protected]/
> 
> v4:
>     https://lore.kernel.org/linux-mm/20260421125508.2317429-1-
> [email protected]/
> 
> v3:
>     https://lore.kernel.org/linux-mm/20260408031615.1831922-1-
> [email protected]/
> 
> [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/rppt/memblock.git/log/?h=k
> ernelcore-mirror
> [2] https://sashiko.dev/#/patchset/20260520093457.3719960-3-
> [email protected]
> 
> Yuan Liu (2):
>   mm/memory_hotplug: optimize zone contiguous check when changing pfn
>     range
>   mm/memory_hotplug: improve shrink_zone_span() subsection boundary
>     checks
> 
>  Documentation/mm/physical_memory.rst | 13 +++++++
>  drivers/base/memory.c                |  6 +++
>  include/linux/mmzone.h               | 47 ++++++++++++++++++++++
>  mm/internal.h                        |  8 +---
>  mm/memory_hotplug.c                  | 54 ++++++++++++++------------
>  mm/mm_init.c                         | 58 +++++++++++++++++-----------
>  6 files changed, 132 insertions(+), 54 deletions(-)
> 
> 
> base-commit: b69c2a1fa9beb4c3db24b4013e07f5cb7e7260aa
> --
> 2.47.3