Re: [PATCH v2 17/46] hugetlbfs: do a full walk to check if vma maps a page

[James Houghton]

On Fri, Feb 17, 2023 at 4:29 PM James Houghton <jthoughton@xxxxxxxxxx> wrote:
>
> Because it is safe to do so, do a full high-granularity page table walk
> to check if the page is mapped.
>
> Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
>
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index cfd09f95551b..c0ee69f0418e 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -386,17 +386,24 @@ static void hugetlb_delete_from_page_cache(struct folio *folio)
>  static bool hugetlb_vma_maps_page(struct vm_area_struct *vma,
>                                 unsigned long addr, struct page *page)
>  {
> -       pte_t *ptep, pte;
> +       pte_t pte;
> +       struct hugetlb_pte hpte;
>
> -       ptep = hugetlb_walk(vma, addr, huge_page_size(hstate_vma(vma)));
> -       if (!ptep)
> +       if (hugetlb_full_walk(&hpte, vma, addr))
>                 return false;
>
> -       pte = huge_ptep_get(ptep);
> +       pte = huge_ptep_get(hpte.ptep);
>         if (huge_pte_none(pte) || !pte_present(pte))
>                 return false;
>
> -       if (pte_page(pte) == page)
> +       if (unlikely(!hugetlb_pte_present_leaf(&hpte, pte)))
> +               /*
> +                * We raced with someone splitting us, and the only case
> +                * where this is impossible is when the pte was none.
> +                */
> +               return false;
> +
> +       if (compound_head(pte_page(pte)) == page)
>                 return true;
>
>         return false;
> --
> 2.39.2.637.g21b0678d19-goog
>

I think this patch is actually incorrect.

This function is *supposed* to check if the page is mapped at all in
this VMA, but really we're only checking if the base address of the
page is mapped. If we did the 'hugetlb_vma_maybe_maps_page' approach
that I did previously and returned 'true' if
!hugetlb_pte_present_leaf(), then this code would be correct again.

But what I really think this function should do is just call
page_vma_mapped_walk(). We're sort of reimplementing it here anyway.
Unless someone disagrees, I'll do this for v3.