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    SubjectRe: [PATCH v2 17/46] hugetlbfs: do a full walk to check if vma maps a page
    On Fri, Feb 17, 2023 at 4:29 PM James Houghton <jthoughton@google.com> 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@google.com>
    >
    > 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.

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