Re: [PATCH] hugetlb: detect race if fail to COW

[John Kacur <jkacur@redhat.com>]

On Fri, Nov 18, 2011 at 3:04 PM, Hillf Danton <dhillf@gmail.com> wrote:
> In the error path that we fail to allocate new huge page, before try again, we
> have to check race since page_table_lock is re-acquired.
>
> If racing, our job is done.
>
> Signed-off-by: Hillf Danton <dhillf@gmail.com>
> ---
>
> --- a/mm/hugetlb.c      Fri Nov 18 21:38:30 2011
> +++ b/mm/hugetlb.c      Fri Nov 18 21:48:15 2011
> @@ -2407,7 +2407,14 @@ retry_avoidcopy:
>                                BUG_ON(page_count(old_page) != 1);
>                                BUG_ON(huge_pte_none(pte));
>                                spin_lock(&mm->page_table_lock);
> -                               goto retry_avoidcopy;
> +                               ptep = huge_pte_offset(mm, address & huge_page_mask(h));
> +                               if (likely(pte_same(huge_ptep_get(ptep), pte)))
> +                                       goto retry_avoidcopy;
> +                               /*
> +                                * race occurs while re-acquiring page_table_lock, and
> +                                * our job is done.
> +                                */
> +                               return 0;
>                        }
>                        WARN_ON_ONCE(1);
>                }


I'm not sure about the veracity of the race condition, but you better
do spin_unlock before you return.

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