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Date: Sat, 15 Feb 2014 20:00:03 -0800 (PST)
From: Hugh Dickins <hughd@google.com>
X-X-Sender: hugh@eggly.anvils
To: Weijie Yang <weijie.yang@samsung.com>
cc: riel@redhat.com, Mel Gorman <mgorman@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Minchan Kim <minchan@kernel.org>, weijie.yang.kh@gmail.com,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
Subject: Re: [RFC PATCH] mm/vmscan: remove two un-needed mem_cgroup_page_lruvec()
 call
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On Sun, 16 Feb 2014, Weijie Yang wrote:

> In putback_inactive_pages() and move_active_pages_to_lru(),
> lruvec is already an input parameter and pages are all from this lruvec,
> therefore there is no need to call mem_cgroup_page_lruvec() in loop.
> 
> Signed-off-by: Weijie Yang <weijie.yang@samsung.com>

Looks plausible but I believe it's incorrect.  The lruvec passed in
is the one we took the pages from, but there's a small but real chance
that the page has become uncharged meanwhile, and should now be put back
on the root_mem_cgroup's lruvec instead of the original memcg's lruvec.

Hugh

> ---
>  mm/vmscan.c |    3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a9c74b4..4804fdb 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1393,8 +1393,6 @@ putback_inactive_pages(struct lruvec *lruvec, struct list_head *page_list)
>  			continue;
>  		}
>  
> -		lruvec = mem_cgroup_page_lruvec(page, zone);
> -
>  		SetPageLRU(page);
>  		lru = page_lru(page);
>  		add_page_to_lru_list(page, lruvec, lru);
> @@ -1602,7 +1600,6 @@ static void move_active_pages_to_lru(struct lruvec *lruvec,
>  
>  	while (!list_empty(list)) {
>  		page = lru_to_page(list);
> -		lruvec = mem_cgroup_page_lruvec(page, zone);
>  
>  		VM_BUG_ON_PAGE(PageLRU(page), page);
>  		SetPageLRU(page);
> -- 
> 1.7.10.4
> 
> 
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