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

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Date:   Thu, 8 Jun 2017 16:48:31 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Larry Finger <Larry.Finger@...inger.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: 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