On Thu, Jan 09, 2025 at 05:38:24PM +0800, 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") Signed-off-by: Zhenhua Huang quic_zhenhuah@quicinc.com
Hi Catalin and Anshuman, I have addressed comments so far, please help review. One outstanding point which not finalized is in vmemmap_populate(): how to judge hotplug section. Currently I am using system_state, discussion: https://lore.kernel.org/linux-mm/1515dae4-cb53-4645-8c72-d33b27ede7eb@quicin...
The patch looks fine to me, apart from one nit and a question below:
@@ -1339,9 +1349,27 @@ int arch_add_memory(int nid, u64 start, u64 size, struct mhp_params *params) { int ret, flags = NO_EXEC_MAPPINGS;
- unsigned long start_pfn = PFN_DOWN(start);
- struct mem_section *ms = __pfn_to_section(start_pfn);
VM_BUG_ON(!mhp_range_allowed(start, size, true));
- /* should not be invoked by early section */
- WARN_ON(early_section(ms));
I don't remember the discussion, do we still need this warning here if the sections are not marked as early? I guess we can keep it if one does an arch_add_memory() on an early section.
I think I suggested to use a WARN_ON_ONCE(!present_section()) but I completely forgot the memory hotplug code paths.
- /*
* Disallow BlOCK/CONT mappings if the corresponding size exceeds
Nit: capital L in BlOCK.
Either way,
Reviewed-by: Catalin Marinas catalin.marinas@arm.com