Re: [PATCH v2 1/7] mm: khugepaged: retract_page_tables() use pte_offset_map_rw_nolock()

[Jann Horn <jannh@google.com>]

On Thu, Oct 31, 2024 at 9:14 AM Qi Zheng <zhengqi.arch@bytedance.com> wrote:
> In retract_page_tables(), we may modify the pmd entry after acquiring the
> pml and ptl, so we should also check whether the pmd entry is stable.

Why does taking the PMD lock not guarantee that the PMD entry is stable?

> Using pte_offset_map_rw_nolock() + pmd_same() to do it, and then we can
> also remove the calling of the pte_lockptr().
>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
>  mm/khugepaged.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 6f8d46d107b4b..6d76dde64f5fb 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1721,6 +1721,7 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
>                 spinlock_t *pml;
>                 spinlock_t *ptl;
>                 bool skipped_uffd = false;
> +               pte_t *pte;
>
>                 /*
>                  * Check vma->anon_vma to exclude MAP_PRIVATE mappings that
> @@ -1756,11 +1757,25 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
>                                         addr, addr + HPAGE_PMD_SIZE);
>                 mmu_notifier_invalidate_range_start(&range);
>
> +               pte = pte_offset_map_rw_nolock(mm, pmd, addr, &pgt_pmd, &ptl);
> +               if (!pte) {
> +                       mmu_notifier_invalidate_range_end(&range);
> +                       continue;
> +               }
> +
>                 pml = pmd_lock(mm, pmd);

I don't understand why you're mapping the page table before locking
the PMD. Doesn't that just mean we need more error checking
afterwards?


> -               ptl = pte_lockptr(mm, pmd);
>                 if (ptl != pml)
>                         spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
>
> +               if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) {
> +                       pte_unmap_unlock(pte, ptl);
> +                       if (ptl != pml)
> +                               spin_unlock(pml);
> +                       mmu_notifier_invalidate_range_end(&range);
> +                       continue;
> +               }
> +               pte_unmap(pte);
> +
>                 /*
>                  * Huge page lock is still held, so normally the page table
>                  * must remain empty; and we have already skipped anon_vma
> --
> 2.20.1
>