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

James Houghton
2022-11-02T18:25:13Z

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
>