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

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Date:   Thu, 8 Jun 2017 10:05:57 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     David Rientjes <rientjes@...gle.com>,
        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 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?