Re: [PATCH v2 1/5] mm/sparse-vmemmap: provide generic vmemmap_set_pmd() and vmemmap_check_pmd()

[Mike Rapoport <rppt@kernel.org>]

Hi,

On Sat, Apr 04, 2026 at 08:20:54PM +0800, Muchun Song wrote:
> The two weak functions are currently no-ops on every architecture,
> forcing each platform that needs them to duplicate the same handful
> of lines.  Provide a generic implementation:
> 
> - vmemmap_set_pmd() simply sets a huge PMD with PAGE_KERNEL protection.
> 
> - vmemmap_check_pmd() verifies that the PMD is present and leaf,
>   then calls the existing vmemmap_verify() helper.
> 
> Architectures that need special handling can continue to override the
> weak symbols; everyone else gets the standard version for free.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  mm/sparse-vmemmap.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 6eadb9d116e4..1eb990610d50 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -391,12 +391,17 @@ int __meminit vmemmap_populate_hvo(unsigned long addr, unsigned long end,
>  void __weak __meminit vmemmap_set_pmd(pmd_t *pmd, void *p, int node,
>  				      unsigned long addr, unsigned long next)
>  {
> +	BUG_ON(!pmd_set_huge(pmd, virt_to_phys(p), PAGE_KERNEL));

Do we have to crash the kernel here?
Wouldn't be better to make vmemmap_set_pmd() return error and make
vmemmap_populate_hugepages() fall back to base pages in case
vmemmap_set_pmd() errored?

>  }
>  
>  int __weak __meminit vmemmap_check_pmd(pmd_t *pmd, int node,
>  				       unsigned long addr, unsigned long next)
>  {
> -	return 0;
> +	if (!pmd_leaf(pmdp_get(pmd)))
> +		return 0;
> +	vmemmap_verify((pte_t *)pmd, node, addr, next);
> +
> +	return 1;
>  }
>  
>  int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
> -- 
> 2.20.1
> 

-- 
Sincerely yours,
Mike.