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

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Date:   Thu, 15 Jun 2017 10:32:26 +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 14-06-17 18:12:06, David Rientjes wrote:
> On Thu, 8 Jun 2017, Michal Hocko wrote:
> 
> > 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.
> > 
> 
> Seems to be an issue for i386 and arm with ARM_LPAE.  I'm slightly 
> surprised we can get away with __collapse_huge_page_swapin() for 
> VM_FAULT_RETRY, unless that hasn't been encountered yet.

I do not see what you mean here or how is it related.
__collapse_huge_page_swapin is called outside of
pte_offset_map/pte_unmap section

> I think the cond_resched() in __collapse_huge_page_copy() could be
> done only for !in_atomic() if we choose.

in_atomic() depends on having PREEMPT_COUNT enabled to work properly AFAIR.
I haven't double checked and something might have changed since I've
looked the last time.

-- 
Michal Hocko
SUSE Labs