Re: [PATCH 1/4] mm/sparse-vmemmap: map shared vmemmap tail pages as read-only

[Lance Yang <lance.yang@linux.dev>]

On Mon, Oct 19, 2026 at 05:08:27PM +0800, Muchun Song wrote:
>Both early HugeTLB HVO and device DAX use the common vmemmap population
>path for shared tail mappings. Map these pages read-only so that illegal
>writes fault instead of corrupting metadata shared by multiple tail
>pages.
>
>For early HugeTLB, this moves existing write protection into initial
>population without changing the final mapping. Device DAX gains the same
>protection.
>
>Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>---
> Documentation/mm/vmemmap_dedup.rst | 3 +--
> mm/sparse-vmemmap.c                | 3 +++
> 2 files changed, 4 insertions(+), 2 deletions(-)
>
>diff --git a/Documentation/mm/vmemmap_dedup.rst b/Documentation/mm/vmemmap_dedup.rst
>index 8c287ae3f86c..76f903975a40 100644
>--- a/Documentation/mm/vmemmap_dedup.rst
>+++ b/Documentation/mm/vmemmap_dedup.rst
>@@ -193,5 +193,4 @@ There's no remapping of vmemmap given that device-dax memory is not part of
> System RAM ranges initialized at boot. Thus the tail page deduplication
> happens at a later stage when we populate the sections.
> 
>-Deduplicated tail pages are not mapped read-only. The mapping layout is the same
>-as HugeTLB.
>+The mapping layout is the same as HugeTLB.
>diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
>index 40eba70c0048..ba0496daf4de 100644
>--- a/mm/sparse-vmemmap.c
>+++ b/mm/sparse-vmemmap.c
>@@ -295,6 +295,9 @@ static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in
> 			return NULL;
> 
> 		entry = pfn_pte(PHYS_PFN(__pa(p)), PAGE_KERNEL);
>+		/* Map the shared tail page as read-only to catch illegal writes. */
>+		if (vmemmap_optimizable_pfn(pfn))
>+			entry = pte_wrprotect(entry);

LLM pointed out that an AO MCE before bootmem gather could still reach
TestSetPageHWPoison() on a shared tail, since the head isn't marked
hugetlb yet.

Wouldn't that trigger an Oops ... Anyway, I wouldn't expect to hit that
during boot.

Reviewed-by: Lance Yang <lance.yang@linux.dev>

[...]

Cheers, Lance