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

[David Hildenbrand <david@redhat.com>]

On 13.02.25 18:56, Catalin Marinas wrote:
> On Thu, Feb 13, 2025 at 05:16:37PM +0100, David Hildenbrand wrote:
>> 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.
> 
> I think that's based on the discussion for a previous version assuming
> that the hotplug/unplug sizes are not guaranteed to be symmetric:
> 
> https://lore.kernel.org/lkml/a720aaa5-a75e-481e-b396-a5f2b50ed362@quicinc.com/
> 
 > If that's not the case, we can indeed ignore the SUBSECTION_SIZE> 
altogether and just rely on the start/end of the hotplugged region.

All cases I know about hotunplug system RAM in the same granularity they 
hotplugged (virtio-mem, dax/kmem, dimm, dlpar), and if they wouldn't, 
they wouldn't operate on sub-section sizes either way.

Regarding dax/pmem, I also recall that it happens always in the same 
granularity. If not, it should be fixed: this weird subsection hotplug 
should not make all other hotplug users suffer (e.g., no vmemmap PMD).

What can likely happen (dax/pmem) is that we hotplug something that 
spans part of 128 MiB section (subsections), to then hotplug something 
that spans another part of a 128 MiB section (subsections). 
Hotunplugging either should not hotplug something part of the other 
device (e.g., rip out the vmemmap PMD).

I think this was expressed with:

"However, 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." -- which is what we should focus on.

I thought x86-64 would handle that case; it would surprise me if 
handling between both archs would have to differ in that regard: with 4k 
arm64 we have the same section/subsection sizes as on x86-64.

-- 
Cheers,

David / dhildenb