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    SubjectRe: [PATCH] hugetlb: detect race if fail to COW
    On Fri 18-11-11 23:23:12, Hillf Danton wrote:
    > On Fri, Nov 18, 2011 at 11:07 PM, Michal Hocko <mhocko@suse.cz> wrote:
    > > On Fri 18-11-11 22:04:37, Hillf Danton 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.
    > >
    > > I do not think we can race here because we are serialized by
    > > hugetlb_instantiation_mutex AFAIU. Without this lock, however, we could
    > > fall into avoidcopy and shortcut despite the fact that other thread has
    > > already did the job.
    > >
    > > The mutex usage is not obvious in hugetlb_cow so maybe we want to be
    > > explicit about it (either a comment or do the recheck).
    > >
    >
    > Then the following check is unnecessary, no?

    Hmm, thinking about it some more, I guess we have to recheck because we
    can still race with page migration. So we need you patch.

    Reviewed-by: Michal Hocko <mhocko@suse.cz>

    >
    > Thanks,
    > Hillf
    >
    > /*
    > * Retake the page_table_lock to check for racing updates
    > * before the page tables are altered
    > */
    > spin_lock(&mm->page_table_lock);
    > ptep = huge_pte_offset(mm, address & huge_page_mask(h));
    > if (likely(pte_same(huge_ptep_get(ptep), pte))) {
    > /* Break COW */
    >
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    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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