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

[Catalin Marinas <catalin.marinas@arm.com>]

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@vger.kernel.org> # v5.4+
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
> ---
>  arch/arm64/mm/mmu.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Will, another bug that has been around for ages. Do you want to take it
as a fix?

> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index b4df5bc5b1b8..1dfe1a8efdbe 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1177,8 +1177,11 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>  		struct vmem_altmap *altmap)
>  {
>  	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
> +	/* [start, end] should be within one section */
> +	WARN_ON_ONCE(end - start > PAGES_PER_SECTION * sizeof(struct page));
>  
> -	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
> +	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) ||
> +	    (end - start < PAGES_PER_SECTION * sizeof(struct page)))
>  		return vmemmap_populate_basepages(start, end, node, altmap);
>  	else
>  		return vmemmap_populate_hugepages(start, end, node, altmap);
> -- 
> 2.25.1
> 

-- 
Catalin