linux-kernel - Re: [patch for-4.12] mm, thp: remove cond_resched from __collapse_huge_page

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Message-ID: <20170623132436.GA5314@dhcp22.suse.cz>
Date:   Fri, 23 Jun 2017 15:24:36 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Larry Finger <Larry.Finger@...inger.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [patch for-4.12] mm, thp: remove cond_resched from
 __collapse_huge_page_copy

On Mon 19-06-17 13:43:11, David Rientjes wrote:
> This is a partial revert of commit 338a16ba1549 ("mm, thp: copying user
> pages must schedule on collapse") which added a cond_resched() to
> __collapse_huge_page_copy().
> 
> On x86 with CONFIG_HIGHPTE, __collapse_huge_page_copy is called in atomic
> context and thus scheduling is not possible.  This is only a possible
> config on arm and i386.
> 
> Although need_resched has been shown to be set for over 100 jiffies while
> doing the iteration in __collapse_huge_page_copy, this is better than
> doing
> 
> 	if (in_atomic())
> 		cond_resched()
> 
> to cover only non-CONFIG_HIGHPTE configs.
> 
> Reported-by: Larry Finger <Larry.Finger@...inger.net>
> Signed-off-by: David Rientjes <rientjes@...gle.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  Note: Larry should be back as of June 17 to test if this fixes the
>  reported issue.
> 
>  mm/khugepaged.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -652,7 +652,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
>  			spin_unlock(ptl);
>  			free_page_and_swap_cache(src_page);
>  		}
> -		cond_resched();
>  	}
>  }
>  
> 
> --
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-- 
Michal Hocko
SUSE Labs