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

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

On Thu, Feb 13, 2025 at 01:59:25PM +0100, David Hildenbrand wrote:
> 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?

Up to 2MB blocks only since that's the SUBSECTION_SIZE value. The
vmemmap mapping is also limited to PAGE_SIZE mappings (we could use
contiguous mappings for vmemmap but it's not wired up; I don't think
it's worth the hassle).

-- 
Catalin