Subject: Re: [PATCH v3 1/5] mm/sparse-vmemmap: provide generic vmemmap_set_pmd() and vmemmap_check_pmd()

From: Muchun Song <[email-masked]>
Date: Mon, 1 Jun 2026 20:37:42 +0800

> On Jun 1, 2026, at 20:22, David Hildenbrand (Arm) <[email-masked]> wrote:
> 
> On 6/1/26 10:48, 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 <[email-masked]>
>> ---
>> v2->v3:
>> - Replace BUG_ON() with WARN_ON_ONCE() in vmemmap_set_pmd()
>> ---
>> 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 112ccf9c71ca..99e2be39671b 100644
>> --- a/mm/sparse-vmemmap.c
>> +++ b/mm/sparse-vmemmap.c
>> @@ -386,12 +386,17 @@ 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)
>> {
>> + 	WARN_ON_ONCE(!pmd_set_huge(pmd, virt_to_phys(p), PAGE_KERNEL));
> 
> 
> Not sure if a VM_WARN_ON_ONCE() would be appropriate. (then, we have to move the
> pmd_set_huge() out of the statement).

I think it might be better to keep WARN_ON_ONCE here. This way, we can still
monitor for warnings in production while keeping the code simple.

> 
> Acked-by: David Hildenbrand (Arm) <[email-masked]>

Thanks.

> -- 
> Cheers,
> David