linux-kernel - Re: [PATCH -mmotm-2009-12-10-17-19] Prevent churning of zero page in LRU list.

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Date:	Mon, 28 Dec 2009 11:57:27 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH -mmotm-2009-12-10-17-19] Prevent churning of zero page
 in LRU list.

On Mon, 28 Dec 2009 11:53:15 +0900
Minchan Kim <minchan.kim@...il.com> wrote:

> 
> VM doesn't add zero page to LRU list. 
> It means zero page's churning in LRU list is pointless. 
> 
> As a matter of fact, zero page can't be promoted by mark_page_accessed
> since it doesn't have PG_lru. 
> 
> This patch prevent unecessary mark_page_accessed call of zero page 
> alghouth caller want FOLL_TOUCH. 
> 
> Signed-off-by: Minchan Kim <minchan.kim@...il.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

> ---
>  mm/memory.c |    7 ++++---
>  1 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 09e4b1b..485f727 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1152,6 +1152,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
>  	spinlock_t *ptl;
>  	struct page *page;
>  	struct mm_struct *mm = vma->vm_mm;
> +	int zero_pfn = 0;
>  
>  	page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
>  	if (!IS_ERR(page)) {
> @@ -1196,15 +1197,15 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
>  
>  	page = vm_normal_page(vma, address, pte);
>  	if (unlikely(!page)) {
> -		if ((flags & FOLL_DUMP) ||
> -		    !is_zero_pfn(pte_pfn(pte)))
> +		zero_pfn = is_zero_pfn(pte_pfn(pte));
> +		if ((flags & FOLL_DUMP) || !zero_pfn )
>  			goto bad_page;
>  		page = pte_page(pte);
>  	}
>  
>  	if (flags & FOLL_GET)
>  		get_page(page);
> -	if (flags & FOLL_TOUCH) {
> +	if (flags & FOLL_TOUCH && !zero_pfn) {
>  		if ((flags & FOLL_WRITE) &&
>  		    !pte_dirty(pte) && !PageDirty(page))
>  			set_page_dirty(page);
> -- 
> 1.5.6.3
> 
> 
> -- 
> Kind regards,
> Minchan Kim
> 
> --
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> 

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