Re: [PATCH v9 0/2] mm/memory_hotplug: optimize zone contiguous check when changing pfn range

[David Hildenbrand (Arm)]

On 9/14/26 09:29, Yuan Liu wrote:
> 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.
> 
> 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 whose memory map has been
> initialized and for which pfn_to_online_page() succeeds.
> 
> For early boot memory, pages_with_online_memmap is calculated in
> memmap_init_zone_range(). PFNs initialized by memmap_init_range() are
> included in pages_with_online_memmap, and hole PFNs for which
> pfn_to_online_page() succeeds are also counted in
> init_unavailable_range(). For hotplugged memory,
> pages_with_online_memmap is updated through adjust_present_page_count(),
> which is called during memory online and offline operations. When
> spanned_pages == pages_with_online_memmap, every PFN in the zone span
> has a valid memmap entry, so pfn_to_page() can be called for any PFN
> within the zone span without an additional pfn_valid() check.
> 
> The counter may temporarily undercount when pages with an online
> memory map exist outside the current zone span. This can only happen
> during boot, when initializing the memory map of pages that do not
> fall into any zone span. Growing the zone to cover such pages and
> later shrinking it back may result in a value that is too small.
> This is safe, as 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 2.
> 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
> ==============
> Patch 1 makes shrink_zone_span() more robust when memory/hole boundary
> falls within a subsection. It checks the full subsection range to avoid
> incorrectly shrinking the zone span during memory unplug.
> 
> Patch 2 introduces pages_with_online_memmap to replace
> pageblock-by-pageblock scans across the entire zone for zone contiguity
> checks.

Sashiko seems to be heavy. I'll wait some more to get more review feedback (and
an Ack from Mike on the mm-init things) before I'll pick this up.

-- 
Cheers,

David