Re: [PATCH RFC 09/10] mm/hugetlb: Make hugetlb_fault() RCU-safe

[James Houghton <jthoughton@google.com>]

On Sun, Oct 30, 2022 at 2:30 PM Peter Xu <peterx@redhat.com> wrote:
>
> RCU makes sure the pte_t* won't go away from under us.  Please refer to the
> comment above huge_pte_offset() for more information.

Thanks for this series, Peter! :)

>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  mm/hugetlb.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 5dc87e4e6780..6d336d286394 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5822,6 +5822,8 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>         int need_wait_lock = 0;
>         unsigned long haddr = address & huge_page_mask(h);
>
> +       /* For huge_pte_offset() */
> +       rcu_read_lock();
>         ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
>         if (ptep) {
>                 /*
> @@ -5830,13 +5832,15 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>                  * not actually modifying content here.
>                  */
>                 entry = huge_ptep_get(ptep);
> +               rcu_read_unlock();
>                 if (unlikely(is_hugetlb_entry_migration(entry))) {
>                         migration_entry_wait_huge(vma, ptep);

ptep is used here (and we dereference it in
`__migration_entry_wait_huge`), so this looks unsafe to me. A simple
way to fix this would be to move the migration entry check after the
huge_pte_alloc call.

- James

>                         return 0;
>                 } else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
>                         return VM_FAULT_HWPOISON_LARGE |
>                                 VM_FAULT_SET_HINDEX(hstate_index(h));
> -       }
> +       } else
> +               rcu_read_unlock();
>
>         /*
>          * Serialize hugepage allocation and instantiation, so that we don't
> --
> 2.37.3
>