Re: [PATCH v9] arm64: mm: Populate vmemmap at the page level if not section aligned

[Oscar Salvador]

On Tue, Mar 04, 2025 at 03:27:00PM +0800, Zhenhua Huang wrote:
> On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set
> to 27, making one section 128M. The related page struct which vmemmap
> points to is 2M then.
> Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
> vmemmap to populate at the PMD section level which was suitable
> initially since hot plug granule is always one section(128M). However,
> commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
> existing arm64 assumptions.
> 
> The first problem is that if start or end is not aligned to a section
> boundary, such as when a subsection is hot added, populating the entire
> section is wasteful.
> 
> The next problem is if we hotplug something that spans part of 128 MiB
> section (subsections, let's call it memblock1), and then hotplug something
> that spans another part of a 128 MiB section(subsections, let's call it
> memblock2), and subsequently unplug memblock1, vmemmap_free() will clear
> the entire PMD entry which also supports memblock2 even though memblock2
> is still active.
> 
> Assuming hotplug/unplug sizes are guaranteed to be symmetric. Do the
> fix similar to x86-64: populate to pages levels if start/end is not aligned
> with section boundary.
> 
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.4+
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Zhenhua Huang <quic_zhenhuah@xxxxxxxxxxx>

Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>


-- 
Oscar Salvador
SUSE Labs