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

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Date:   Wed, 14 Jun 2017 18:12:06 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
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 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 think the 
cond_resched() in __collapse_huge_page_copy() could be done only for 
!in_atomic() if we choose.