riscv: mm: exclude invalid THP PMDs from page table check

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.

Link: https://lore.kernel.org/20260523042052.35476-1-cuiyunhui@bytedance.com
Fixes: 3fee229a8eb9 ("riscv/mm: enable ARCH_SUPPORTS_PAGE_TABLE_CHECK")
Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: tongtiangen <tongtiangen@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 8 insertions, 1 deletions

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 5d5756bda82e..1a9c14d71df4 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -986,7 +986,14 @@ static inline bool pte_user_accessible_page(struct mm_struct *mm, unsigned long
 
 static inline bool pmd_user_accessible_page(struct mm_struct *mm, unsigned long addr, 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(struct mm_struct *mm, unsigned long addr, pud_t pud)