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@xxxxxxxxxxxxxxx> # v5.4+
Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Zhenhua Huang <quic_zhenhuah@xxxxxxxxxxx>
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@xxxxxxxxxxx/
> 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
