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

[Zhenhua Huang <quic_zhenhuah@quicinc.com>]

On 2025/2/14 2:20, David Hildenbrand wrote:
> 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.
> 

Thanks David and Catalin. From your discussion, I understand that 
hotplug/unplug sizes are guaranteed to be symmetric ? Therefore, it 
should be straightforward to populate to base pages if (end - start < 
PAGES_PER_SECTION * sizeof(struct page)) ? I will write patch and verify.
Please correct me if my understanding is incorrect.