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    SubjectRe: Sleeping BUG in khugepaged for i586
    On Wed 07-06-17 13:56:01, David Rientjes wrote:
    > On Wed, 7 Jun 2017, Vlastimil Babka wrote:
    >
    > > >> Hmm I'd expect such spin lock to be reported together with mmap_sem in
    > > >> the debugging "locks held" message?
    > > >
    > > > My bisection of the problem is about half done. My latest good version is commit
    > > > 7b8cd33 and the latest bad one is 2ea659a. Only about 7 steps to go.
    > >
    > > Hmm, your bisection will most likely just find commit 338a16ba15495
    > > which added the cond_resched() at mm/khugepaged.c:655. CCing David who
    > > added it.
    > >
    >
    > I agree it's probably going to bisect to 338a16ba15495 since it's the
    > cond_resched() at the line number reported, but I think there must be
    > something else going on. I think the list of locks held by khugepaged is
    > correct because it matches with the implementation. The preempt_count(),
    > as suggested by Andrew, does not. If this is reproducible, I'd like to
    > know what preempt_count() is.

    collapse_huge_page
    pte_offset_map
    kmap_atomic
    kmap_atomic_prot
    preempt_disable
    __collapse_huge_page_copy
    pte_unmap
    kunmap_atomic
    __kunmap_atomic
    preempt_enable

    I suspect, so cond_resched seems indeed inappropriate on 32b systems.

    --
    Michal Hocko
    SUSE Labs

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