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    Subject[PATCH] hugetlb: detect race if fail to COW
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    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);
    }

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