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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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: Alexandre Ghiti <alex@ghiti.fr>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Vivian Wang <wangruikang@iscas.ac.cn>
Cc: Yaxing Guo <guoyaxing@bosc.ac.cn>
Cc: <stable@vger.kernel.org>

0 files changed, 0 insertions, 0 deletions