linux-kernel - Re: [PATCH 02/13] mm: use page_off

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Date:   Fri, 18 Sep 2020 09:37:00 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Chris Down <chris@...isdown.name>,
        Yafang Shao <laoar.shao@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Huang Ying <ying.huang@...el.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Minchan Kim <minchan@...nel.org>,
        Jaewon Kim <jaewon31.kim@...sung.com>, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/13] mm: use page_off_lru()

On Thu 17-09-20 21:00:40, Yu Zhao wrote:
> This patch replaces the only open-coded __ClearPageActive() with
> page_off_lru(). There is no open-coded __ClearPageUnevictable()s.
> 
> Before this patch, we have:
> 	__ClearPageActive()
> 	add_page_to_lru_list()
> 
> After this patch, we have:
> 	page_off_lru()
> 		if PageUnevictable()
> 			__ClearPageUnevictable()
> 		else if PageActive()
> 			__ClearPageActive()
> 	add_page_to_lru_list()
> 
> Checking PageUnevictable() shouldn't be a problem because these two
> flags are mutually exclusive. Leaking either will trigger bad_page().

I am sorry but the changelog is really hard to grasp. What are you
trying to achieve, why and why it is safe. This should be a general
outline for any patch. I have already commented on the previous patch
and asked you for the explanation why removing __ClearPageActive from
this path is desirable and safe. I have specifically asked to clarify
the compound page situation as that is using its oen destructor in the
freeing path and that might result in page_off_lru to be not called.
 
> Signed-off-by: Yu Zhao <yuzhao@...gle.com>
> ---
>  mm/vmscan.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 503fc5e1fe32..f257d2f61574 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1845,7 +1845,6 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
>  	int nr_pages, nr_moved = 0;
>  	LIST_HEAD(pages_to_free);
>  	struct page *page;
> -	enum lru_list lru;
>  
>  	while (!list_empty(list)) {
>  		page = lru_to_page(list);
> @@ -1860,14 +1859,11 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
>  		lruvec = mem_cgroup_page_lruvec(page, pgdat);
>  
>  		SetPageLRU(page);
> -		lru = page_lru(page);
> -
>  		add_page_to_lru_list(page, lruvec, lru);
>  
>  		if (put_page_testzero(page)) {
>  			__ClearPageLRU(page);
> -			__ClearPageActive(page);
> -			del_page_from_lru_list(page, lruvec, lru);
> +			del_page_from_lru_list(page, lruvec, page_off_lru(page));
>  
>  			if (unlikely(PageCompound(page))) {
>  				spin_unlock_irq(&pgdat->lru_lock);
> -- 
> 2.28.0.681.g6f77f65b4e-goog

-- 
Michal Hocko
SUSE Labs