Re: Sleeping BUG in khugepaged for i586

Michal Hocko
2017-06-08T14:48:36Z

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