Re: [PATCH hotfix] mm: fix pmd_special() fallback to observe huge_zero

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/29/26 07:54, Lance Yang wrote:
> 
> On Tue, Apr 28, 2026 at 10:08:37PM -0700, Hugh Dickins wrote:
>> On x86 32-bit with THP enabled, zap_huge_pmd() is seen to generate a
>> "WARNING: mm/memory.c:735 at __vm_normal_page+0x6a/0x7d", from the
>> VM_WARN_ON_ONCE(is_zero_pfn(pfn) || is_huge_zero_pfn(pfn)); followed
>> by "BUG: Bad rss-counter state"s, then later "BUG: Bad page state"s
>> when reclaim gets to call shrink_huge_zero_folio_scan().
> 
> Good catch!
> 
>> It's as if the _PAGE_SPECIAL bit never got set in the huge_zero pmd:
>> and indeed, whereas pte_special() and pte_mkspecial() are subject to a
>> dedicated CONFIG_ARCH_HAS_PTE_SPECIAL, pmd_special() and pmd_mkspecial()
>> are subject to CONFIG_ARCH_SUPPORTS_PMD_PFNMAP, which is never enabled
>> on any 32-bit architecture.
>>
>> Add CONFIG_ARCH_HAS_PMD_SPECIAL?  Perhaps; but I think it's better just
>> to observe the huge_zero pmd in the fallback version of pmd_special().
>>
>> Fixes: d80a9cb1a64a ("mm/huge_memory: add and use normal_or_softleaf_folio_pmd()")

Likely it should be

	Fixes: d82d09e48219 ("mm/huge_memory: mark PMD mappings of the huge zero folio special")

Because vm_normal_page_pmd() would return the wrong thing.

But I am surprised that we didn't run into the

	VM_WARN_ON_ONCE(is_zero_pfn(pfn) || is_huge_zero_pfn(pfn));

earlier. 	

>> Signed-off-by: Hugh Dickins <hughd@google.com>
>> ---
>> include/linux/mm.h | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 0b776907152e..3b02ac43bcb7 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -3422,7 +3422,7 @@ static inline pte_t pte_mkspecial(pte_t pte)
>> #ifndef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
>> static inline bool pmd_special(pmd_t pmd)
>> {
>> -	return false;
>> +	return is_huge_zero_pmd(pmd);
>> }
> 
> Emm ... feels a bit odd to me ...

Agreed. But it could be a temporary fix until we fixed up relevant architectures.

> 
> On these configs pmd_mkspecial() is still a no-op, so pmd_special()
> would no longer really mean that the PMD was made special :)
> 
> Could we handle the huge zero PMD in vm_normal_page_pmd() instead?

That adds unnecessary checks for architectures that properly implement pmd_special.

pmd_special() should be fixed longterm on architectures that support THP
and CONFIG_ARCH_HAS_PTE_SPECIAL. It should not be glued to CONFIG_ARCH_SUPPORTS_PMD_PFNMAP.


arch/arc/Kconfig:       select HAVE_ARCH_TRANSPARENT_HUGEPAGE if ARC_MMU_V4
arch/arm/Kconfig:       select HAVE_ARCH_TRANSPARENT_HUGEPAGE if ARM_LPAE
arch/arm64/Kconfig:     select HAVE_ARCH_TRANSPARENT_HUGEPAGE
arch/loongarch/Kconfig: select HAVE_ARCH_TRANSPARENT_HUGEPAGE
arch/mips/Kconfig:      select HAVE_ARCH_TRANSPARENT_HUGEPAGE if CPU_SUPPORTS_HUGEPAGES
arch/powerpc/platforms/Kconfig.cputype:       select HAVE_ARCH_TRANSPARENT_HUGEPAGE
arch/riscv/Kconfig:     select HAVE_ARCH_TRANSPARENT_HUGEPAGE if 64BIT && MMU
arch/s390/Kconfig:      select HAVE_ARCH_TRANSPARENT_HUGEPAGE
arch/sparc/Kconfig:     select HAVE_ARCH_TRANSPARENT_HUGEPAGE
arch/x86/Kconfig:       select HAVE_ARCH_TRANSPARENT_HUGEPAGE

arch/arc/Kconfig:       select ARCH_HAS_PTE_SPECIAL
arch/arm/Kconfig:       select ARCH_HAS_PTE_SPECIAL if ARM_LPAE
arch/arm64/Kconfig:     select ARCH_HAS_PTE_SPECIAL
arch/loongarch/Kconfig: select ARCH_HAS_PTE_SPECIAL
arch/mips/Kconfig:      select ARCH_HAS_PTE_SPECIAL if !(32BIT && CPU_HAS_RIXI)
arch/parisc/Kconfig:    select ARCH_HAS_PTE_SPECIAL
arch/powerpc/Kconfig:   select ARCH_HAS_PTE_SPECIAL
arch/riscv/Kconfig:     select ARCH_HAS_PTE_SPECIAL
arch/s390/Kconfig:      select ARCH_HAS_PTE_SPECIAL
arch/sh/Kconfig:        select ARCH_HAS_PTE_SPECIAL
arch/sparc/Kconfig:     select ARCH_HAS_PTE_SPECIAL
arch/x86/Kconfig:       select ARCH_HAS_PTE_SPECIAL

That's a bit of work given that only arm64, powerpc (64), riscv and x86 (64)
properly implement pmd_special().


So I think Hugh's patch here makes sense for now.

-- 
Cheers,

David