Re: [PATCH v5] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

[Catalin Marinas <catalin.marinas@arm.com>]

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@quicinc.com/

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>