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

[Oscar Salvador]

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.
> 
> 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@xxxxxxxxxx>
> Closes: https://lore.kernel.org/r/74a75b59-2e13-3985-ee99-d5521f39df2a@xxxxxxxxxx
> Reported-by: Bibo Mao <maobibo@xxxxxxxxxxx>
> Closes: https://lore.kernel.org/r/20260430041121.2839350-1-maobibo@xxxxxxxxxxx
> Debugged-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>
> Tested-by: Bibo Mao <maobibo@xxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>

 

-- 
Oscar Salvador
SUSE Labs