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

On Mon, Jun 01, 2026 at 08:37:42PM +0800, Muchun Song wrote:
> 
> 
> > On Jun 1, 2026, at 20:22, David Hildenbrand (Arm) <[email protected]> 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 protected]>
> >> ---
> >> 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.

IIRC there was a discussion about this some time ago, given the fact that quite
some people out there tend to have 'panic_on_warn', but then again, if we fail to
set a PMD here it means something pretty nasty happened, so I am ok with that.

 

-- 
Oscar Salvador
SUSE Labs