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

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 23 May 2026 12:20:52 +0800 Yunhui Cui <cuiyunhui@bytedance.com> wrote:

> 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.

Thanks.  This seems to have slipped through cracks.

AI review appears to have found a couple of related and serious issues
in this code.

	https://sashiko.dev/#/patchset/20260523042052.35476-1-cuiyunhui@bytedance.com

perhaps you have time to take a look?

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

I'm not even slightly a riscv maintainer, but I'll queue this up for
some linux-next testing and so I can keep an eye on the issue, thanks.