Re: [PATCH] mm: fix __vm_normal_page() to handle missing support for pmd_special()/pud_special()

[Lorenzo Stoakes <ljs@kernel.org>]

On Thu, Apr 30, 2026 at 01:31:22PM +0200, David Hildenbrand (Arm) 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().
>
> 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.

Ah damn. I wonder if it's really a combination of 'supports THP' and 'has a
spare software defined bit free in PTE'?

In any case obviously have to fix this.

>
> While the problem was exposed through commit d80a9cb1a64a ("mm/huge_memory:
> add and use normal_or_softleaf_folio_pmd()"), it was an oversight in commit
> af38538801c6 ("mm/memory: factor out common code from vm_normal_page_*()")
> and would result in other problems:
> * huge zero folio accounted in smaps, pagemap (PAGE_IS_FILE) and
>   numamaps as file-backed THP
> * folio_walk_start() returning the folio even without FW_ZEROPAGE set.
>   Callers seem to tolerate that, though.
>
> ... and triggering the VM_WARN_ON_ONE(), although never reported so far.
>
> To fix it, teach vm_normal_page_pmd()/vm_normal_page_pud() to consider
> whether pmd_special/pud_special is actually implemented.
>
> Fixes: af38538801c6 ("mm/memory: factor out common code from vm_normal_page_*()")
> Reported-by: Hugh Dickins <hughd@google.com>
> Closes: https://lore.kernel.org/r/74a75b59-2e13-3985-ee99-d5521f39df2a@google.com
> Reported-by: Bibo Mao <maobibo@loongson.cn>
> Closes: https://lore.kernel.org/r/20260430041121.2839350-1-maobibo@loongson.cn
> Debugged-by: Hugh Dickins <hughd@google.com>
> Reviewed-by: Lance Yang <lance.yang@linux.dev>
> Tested-by: Bibo Mao <maobibo@loongson.cn>
> Cc: stable@vger.kernel.org
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

This LGTM, so:

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>

> ---
> This is an alternative to Hugh's patch, whereby we leave pmd_special()
> be a NOP and instead teach __vm_normal_page() about lack of support for
> pmd_special/pud_special.
> ---
>  mm/memory.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 7322a40e73b9..4d84976fc7f4 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -612,6 +612,21 @@ static void print_bad_page_map(struct vm_area_struct *vma,
>  	dump_stack();
>  	add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
>  }
> +
> +static inline bool pgtable_level_has_pxx_special(enum pgtable_level level)
> +{
> +	switch (level) {
> +	case PGTABLE_LEVEL_PTE:
> +		return IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL);
> +	case PGTABLE_LEVEL_PMD:
> +		return IS_ENABLED(CONFIG_ARCH_SUPPORTS_PMD_PFNMAP);
> +	case PGTABLE_LEVEL_PUD:
> +		return IS_ENABLED(CONFIG_ARCH_SUPPORTS_PUD_PFNMAP);
> +	default:
> +		return false;
> +	}
> +}
> +
>  #define print_bad_pte(vma, addr, pte, page) \
>  	print_bad_page_map(vma, addr, pte_val(pte), page, PGTABLE_LEVEL_PTE)
>
> @@ -684,7 +699,7 @@ static inline struct page *__vm_normal_page(struct vm_area_struct *vma,
>  		unsigned long addr, unsigned long pfn, bool special,
>  		unsigned long long entry, enum pgtable_level level)
>  {
> -	if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)) {
> +	if (pgtable_level_has_pxx_special(level)) {
>  		if (unlikely(special)) {
>  #ifdef CONFIG_FIND_NORMAL_PAGE
>  			if (vma->vm_ops && vma->vm_ops->find_normal_page)
> @@ -699,8 +714,9 @@ static inline struct page *__vm_normal_page(struct vm_area_struct *vma,
>  			return NULL;
>  		}
>  		/*
> -		 * With CONFIG_ARCH_HAS_PTE_SPECIAL, any special page table
> -		 * mappings (incl. shared zero folios) are marked accordingly.
> +		 * With working pte_special()/pmd_special()..., any special page
> +		 * table mappings (incl. shared zero folios) are marked
> +		 * accordingly.
>  		 */
>  	} else {
>  		if (unlikely(vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))) {
>
> ---
>
> base-commit: d94322006a51b522dd361128a450bf9e75aad889
>
> change-id: 20260430-pmd_special-610dbdd8ac3c
>
> --
>
> Cheers,
>
> David
>