Re: [PATCH RFC 07/10] mm/hugetlb: Make hugetlb_follow_page_mask() RCU-safe

[James Houghton <jthoughton@google.com>]

On Sun, Oct 30, 2022 at 2:29 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.
>
> 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 9869c12e6460..85214095fb85 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6229,10 +6229,12 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
>         if (WARN_ON_ONCE(flags & FOLL_PIN))
>                 return NULL;
>
> +       /* For huge_pte_offset() */
> +       rcu_read_lock();
>  retry:
>         pte = huge_pte_offset(mm, haddr, huge_page_size(h));
>         if (!pte)
> -               return NULL;
> +               goto out_rcu;
>
>         ptl = huge_pte_lock(h, mm, pte);

Just to make sure -- this huge_pte_lock doesn't count as "blocking"
(for the purposes of what is allowed in an RCU read-side critical
section), right? If so, great! But I think we need to call
`rcu_read_unlock` before entering `__migration_entry_wait_huge`, as
that function really can block.

- James

>         entry = huge_ptep_get(pte);
> @@ -6266,6 +6268,8 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
>         }
>  out:
>         spin_unlock(ptl);
> +out_rcu:
> +       rcu_read_unlock();
>         return page;
>  }
>
> --
> 2.37.3
>