Date: Thu, 8 Jun 2017 22:18:24 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: David Rientjes <rientjes@google.com>, Vlastimil Babka <vbabka@suse.cz>,
        Larry Finger <Larry.Finger@lwfinger.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org
Subject: Re: Sleeping BUG in khugepaged for i586
Message-ID: <20170608201822.GA5535@dhcp22.suse.cz>
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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