linux-kernel - Re: Sleeping BUG in khugepaged for i586

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Date:   Fri, 23 Jun 2017 14:08:12 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Larry Finger <Larry.Finger@...inger.net>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: Sleeping BUG in khugepaged for i586

On Thu 08-06-17 16:48:31, Michal Hocko wrote:
> 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.

The code still seems to be in the mmotm tree. Are there any plans to fix
this or drop the patch?
-- 
Michal Hocko
SUSE Labs