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

[David Hildenbrand <david@redhat.com>]

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@vger.kernel.org> # v5.4+
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.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