Re: [RFC PATCH] LoongArch: mm: avoid backing vmemmap beyond max_pfn

[Muchun Song]

> On Oct 21, 2026, at 14:21, Wentao Guan <guanwentao@xxxxxxxxxxxxx> wrote:
> 
> A PMD maps 32MiB of vmemmap on LoongArch with 16KiB base pages, while a
> 512MiB section only needs 2MiB.  vmemmap_populate() is called for each
> section and vmemmap_populate_hugepages() installs a PMD mapping for the
> whole PMD, so the last PMD also backs struct pages for PFNs beyond
> max_pfn.
> 
> On a 34GiB system 160MiB of vmemmap is mapped although 136MiB is
> needed.  The extra 24MiB describes PFNs that do not exist.

Your commit messages are very detailed, but I don't really understand the
problem you're trying to solve. What issue did you run into, and what was
the resulting impact?

Thanks,
Muchun

> 
> Other architectures handle this differently.  On x86, arm64 with 4KiB
> pages, and riscv, the PMD covers exactly one section's vmemmap, so the
> huge page path never maps beyond the populated range; x86 tracks
> partially used PMDs with PAGE_UNUSED for memory hotremove.  arm64 uses
> base pages for 16KiB and 64KiB kernels.  s390 deliberately keeps using
> 1MiB frames even for partially used PMDs because vmemmap_populate() is
> called per section and falling back would add page tables.  powerpc
> radix falls back to base pages only when the altmap cannot back a full
> PMD or the allocation fails, and powerpc hash maps the vmemmap with the
> linear mapping page size, which can exceed a section as well.
> 
> With 16KiB pages the LoongArch PMD is 16 times the vmemmap of one
> section.  Moving the whole vmemmap to base pages, as arm64 does for
> 16KiB and 64KiB, would also remove the tail, but it would give up PMD
> mappings for the 128MiB that is fully populated.  Use base pages only
> for the PMD that extends beyond pfn_to_page(max_pfn) and keep huge
> mappings for the rest.
> 
> The altmap case is left alone because max_pfn does not describe it.
> 
> Signed-off-by: Wentao Guan <guanwentao@xxxxxxxxxxxxx>
> ---
> arch/loongarch/mm/init.c | 5 +++++
> 1 file changed, 5 insertions(+)
> 
> diff --git a/arch/loongarch/mm/init.c b/arch/loongarch/mm/init.c
> index 4b46c5d30708d..bb7a17bdb0e51 100644
> --- a/arch/loongarch/mm/init.c
> +++ b/arch/loongarch/mm/init.c
> @@ -146,6 +146,11 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end,
> #if CONFIG_PGTABLE_LEVELS == 2
> 	return vmemmap_populate_basepages(start, end, node, NULL);
> #else
> + 	unsigned long vmemmap_end = (unsigned long)pfn_to_page(max_pfn);
> +
> + 	if (!altmap && (start & PMD_MASK) + PMD_SIZE > vmemmap_end)
> + 		return vmemmap_populate_basepages(start, end, node, NULL);
> +
> 	return vmemmap_populate_hugepages(start, end, node, NULL);
> #endif
> }
> -- 
> 2.39.5
>