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

[yunhui cui]

Hi Paul,

On Sat, May 23, 2026 at 7:10 AM Paul Walmsley <pjw at kernel.org> wrote:
>
> On Fri, 15 May 2026, Yunhui Cui 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.
> >
> > 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>
>
> Thanks for the patch, but it doesn't apply as-is.  Could you fix it and
> resend?

Thanks for the heads-up. I have fixed the apply issue and resent it here:
https://lore.kernel.org/all/20260523042052.35476-1-cuiyunhui@bytedance.com/

>
>
> - Paul

 Thanks,
 Yunhui