Re: [PATCH 2/6] mm/hugetlb: Fix uffd-wp bit lost when unsharing happens

Mike Kravetz
2023-04-14T22:20:04Z

On 04/13/23 19:11, Peter Xu wrote:
> When we try to unshare a pinned page for a private hugetlb, uffd-wp bit can
> get lost during unsharing. Fix it by carrying it over.
>
> This should be very rare, only if an unsharing happened on a private
> hugetlb page with uffd-wp protected (e.g. in a child which shares the same
> page with parent with UFFD_FEATURE_EVENT_FORK enabled).
>
> Cc: linux-stable <stable@vger.kernel.org>
> Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
> Reported-by: Mike Kravetz <mike.kravetz@oracle.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> mm/hugetlb.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 7320e64aacc6..083aae35bff8 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5637,13 +5637,16 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
> spin_lock(ptl);
> ptep = hugetlb_walk(vma, haddr, huge_page_size(h));
> if (likely(ptep && pte_same(huge_ptep_get(ptep), pte))) {
> + pte_t newpte = make_huge_pte(vma, &new_folio->page, !unshare);
> +
> /* Break COW or unshare */
> huge_ptep_clear_flush(vma, haddr, ptep);
> mmu_notifier_invalidate_range(mm, range.start, range.end);
> page_remove_rmap(old_page, vma, true);
> hugepage_add_new_anon_rmap(new_folio, vma, haddr);
> - set_huge_pte_at(mm, haddr, ptep,
> - make_huge_pte(vma, &new_folio->page, !unshare));
> + if (huge_pte_uffd_wp(pte))
> + newpte = huge_pte_mkuffd_wp(newpte);
> + set_huge_pte_at(mm, haddr, ptep, newpte);
> folio_set_hugetlb_migratable(new_folio);
> /* Make the old page be freed below */
> new_folio = page_folio(old_page);
> --
> 2.39.1
>

Thanks! Looks good,

Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

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Mike Kravetz