[PATCH] riscv: mm: exclude invalid THP PMDs from page table check

[Yunhui Cui]

RISC-V THP splitting uses a temporary invalid PMD state where
pmd_mkinvalid() clears _PAGE_PRESENT and _PAGE_PROT_NONE but leaves
_PAGE_LEAF set so the MM code can still recognize the PMD as a THP split
in-progress entry.

That temporary state no longer describes a user-accessible mapping, but
page_table_check currently treats it as one because the RISC-V PMD
user-accessibility test only checks whether the PMD is a leaf and has
user permissions.

As a result, when a PMD-sized anonymous THP is split during a COW fault,
page_table_check can account the invalid intermediate PMD as a live PMD
mapping, and then account the replacement PTE mappings again when the
split installs the PTE table. This leaves stale PMD accounting behind and
later triggers page_table_check failures such as a non-zero
anon_map_count when the folio is freed.

Fix this by tightening pmd_user_accessible_page() so PMD page-table-check
accounting only considers leaf PMDs that still carry either
_PAGE_PRESENT or _PAGE_PROT_NONE. This preserves the THP split semantics
required by the MM code while preventing page_table_check from treating
invalid split PMDs as live user mappings.

With CONFIG_PAGE_TABLE_CHECK=y and CONFIG_PAGE_TABLE_CHECK_ENFORCED=y,
tools/testing/selftests/mm/cow completes successfully on RISC-V after
this change.

Fixes: 3fee229a8eb9 ("riscv/mm: enable ARCH_SUPPORTS_PAGE_TABLE_CHECK")
Cc: stable at vger.kernel.org
Signed-off-by: Yunhui Cui <cuiyunhui at bytedance.com>
---
 arch/riscv/include/asm/pgtable.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 1725f0861f6c7..88c599fda5779 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -1209,7 +1209,14 @@ static inline bool pte_user_accessible_page(pte_t pte)
 
 static inline bool pmd_user_accessible_page(pmd_t pmd)
 {
-	return pmd_leaf(pmd) && pmd_user(pmd);
+	/*
+	 * page_table_check() must ignore THP split invalidation entries created by
+	 * pmd_mkinvalid(). These retain _PAGE_LEAF so pmd_present()/pmd_leaf() stay
+	 * true during the split, but they no longer describe a user-accessible
+	 * mapping once both _PAGE_PRESENT and _PAGE_PROT_NONE are cleared.
+	 */
+	return (pmd_val(pmd) & (_PAGE_PRESENT | _PAGE_PROT_NONE)) &&
+		(pmd_val(pmd) & _PAGE_LEAF) && pmd_user(pmd);
 }
 
 static inline bool pud_user_accessible_page(pud_t pud)
-- 
2.39.5