Re: Sleeping BUG in khugepaged for i586

Michal Hocko
2017-06-08T20:18:29Z

On Thu 08-06-17 10:05:57, Matthew Wilcox wrote:
> On Thu, Jun 08, 2017 at 04:48:31PM +0200, Michal Hocko wrote:
> > On Wed 07-06-17 13:56:01, David Rientjes wrote:
> > > 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.
>
> Then why doesn't it trigger on 64-bit systems too?
>
> #ifndef ARCH_HAS_KMAP
> ...
> static inline void *kmap_atomic(struct page *page)
> {
> preempt_disable();
> pagefault_disable();
> return page_address(page);
> }
> #define kmap_atomic_prot(page, prot) kmap_atomic(page)
>
>
> ... oh, wait, I see. Because pte_offset_map() doesn't call kmap_atomic()
> on 64-bit. Indeed, it doesn't necessarily call kmap_atomic() on 32-bit
> either; only with CONFIG_HIGHPTE enabled. How much of a performance
> penalty would it be to call kmap_atomic() unconditionally on 64 bit to
> make sure that this kind of problem doesn't show on 32-bit systems only?

I am not sure I understand why would we map those pages in 64b systems?
We can access them directly.

--
Michal Hocko
SUSE Labs