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 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.

-- 
Catalin