linux-kernel - Re: [PATCH v8] arm64: mm: Populate vmemmap at the page level if not section aligned

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Message-ID: <78d55e35-6cda-4f5e-8e52-0a54b1e64592@redhat.com>
Date: Mon, 3 Mar 2025 11:01:24 +0100
From: David Hildenbrand <david@...hat.com>
To: Zhenhua Huang <quic_zhenhuah@...cinc.com>, anshuman.khandual@....com,
 catalin.marinas@....com
Cc: will@...nel.org, ardb@...nel.org, ryan.roberts@....com,
 mark.rutland@....com, joey.gouly@....com, dave.hansen@...ux.intel.com,
 akpm@...ux-foundation.org, chenfeiyang@...ngson.cn, chenhuacai@...nel.org,
 linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, quic_tingweiz@...cinc.com,
 stable@...r.kernel.org
Subject: Re: [PATCH v8] arm64: mm: Populate vmemmap at the page level if not
 section aligned

On 19.02.25 09:40, 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@...r.kernel.org> # v5.4+
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Signed-off-by: Zhenhua Huang <quic_zhenhuah@...cinc.com>
> ---
> Hi Catalin and David,
> Following our latest discussion, I've updated the patch for your review.
> I also removed Catalin's review tag since I've made significant modifications.
>   arch/arm64/mm/mmu.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index b4df5bc5b1b8..de05ccf47f21 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(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)))

Indentation should be

	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) ||
	    (end - start < PAGES_PER_SECTION * sizeof(struct page)))


Acked-by: David Hildenbrand <david@...hat.com>


Thanks!

-- 
Cheers,

David / dhildenb