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

[David Hildenbrand <david@redhat.com>]

On 13.02.25 16:49, Catalin Marinas wrote:
> 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).

But that's messed up, no?

If someone hotplugs a memory section, they have to hotunplug a memory 
section, not parts of it.

That's why x86 does in vmemmap_populate():

if (end - start < PAGES_PER_SECTION * sizeof(struct page))
	err = vmemmap_populate_basepages(start, end, node, NULL);
else if (boot_cpu_has(X86_FEATURE_PSE))
	err = vmemmap_populate_hugepages(start, end, node, altmap);
...

Maybe I'm missing something. Most importantly, why the weird subsection 
stuff is supposed to degrade ordinary hotplug of dimms/virtio-mem etc.

-- 
Cheers,

David / dhildenb