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. 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()") Signed-off-by: Hugh Dickins --- 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); } static inline pmd_t pmd_mkspecial(pmd_t pmd) -- 2.51.0