linux-kernel - Re: [PATCH v6] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

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Message-ID: <9bc91fe3-c590-48e2-b29f-736d0b056c34@redhat.com>
Date: Thu, 13 Feb 2025 13:59:25 +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 v6] arm64: mm: Populate vmemmap/linear at the page level
 for hotplugged sections

On 13.02.25 08:57, 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.
> 
> Considering the vmemmap_free -> unmap_hotplug_pmd_range path, when
> pmd_sect() is true, the entire PMD section is cleared, even if there is
> other effective subsection. For example page_struct_map1 and
> page_strcut_map2 are part of a single PMD entry and they are hot-added
> sequentially. Then page_struct_map1 is removed, vmemmap_free() will clear
> the entire PMD entry freeing the struct page map for the whole section,
> even though page_struct_map2 is still active. Similar problem exists
> with linear mapping as well, for 16K base page(PMD size = 32M) or 64K
> base page(PMD = 512M), their block mappings exceed SUBSECTION_SIZE.
> Tearing down the entire PMD mapping too will leave other subsections
> unmapped in the linear mapping.
> 
> To address the issue, we need to prevent PMD/PUD/CONT mappings for both
> linear and vmemmap for non-boot sections if corresponding size on the
> given base page exceeds SUBSECTION_SIZE(2MB now).
> 
> Cc: <stable@...r.kernel.org> # v5.4+
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Zhenhua Huang <quic_zhenhuah@...cinc.com>

Just so I understand correctly: for ordinary memory-sections-size 
hotplug (NVDIMM, virtio-mem), we still get a large mapping where possible?


-- 
Cheers,

David / dhildenb