Re: [PATCH 01/13] mm: use add_page_to_lru_list()

From: Michal Hocko <mhocko@suse.com>
To: Yu Zhao <yuzhao@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Alex Shi <alex.shi@linux.alibaba.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Roman Gushchin <guro@fb.com>, Shakeel Butt <shakeelb@google.com>,
	Chris Down <chris@chrisdown.name>,
	Yafang Shao <laoar.shao@gmail.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Huang Ying <ying.huang@intel.com>,
	Pankaj Gupta <pankaj.gupta.linux@gmail.com>,
	Matthew Wilcox <willy@infradead.org>,
	Konstantin Khlebnikov <koct9i@gmail.com>,
	Minchan Kim <minchan@kernel.org>,
	Jaewon Kim <jaewon31.kim@samsung.com>,
	cgroups@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 01/13] mm: use add_page_to_lru_list()
Date: Fri, 18 Sep 2020 09:32:40 +0200	[thread overview]
Message-ID: <20200918073240.GD28827@dhcp22.suse.cz> (raw)
In-Reply-To: <20200918030051.650890-2-yuzhao@google.com>

On Thu 17-09-20 21:00:39, Yu Zhao wrote:
> This patch replaces the only open-coded lru list addition with
> add_page_to_lru_list().
> 
> Before this patch, we have:
> 	update_lru_size()
> 	list_move()
> 
> After this patch, we have:
> 	list_del()
> 	add_page_to_lru_list()
> 		update_lru_size()
> 		list_add()
> 
> The only side effect is that page->lru is temporarily poisoned
> after a page is deleted from its old list, which shouldn't be a
> problem.

"because the lru lock is held" right?

Please always be explicit about your reasoning. "It shouldn't be a
problem" without further justification is just pointless for anybody
reading the code later on.

> Signed-off-by: Yu Zhao <yuzhao@google.com>
> ---
>  mm/vmscan.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9727dd8e2581..503fc5e1fe32 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1850,8 +1850,8 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
>  	while (!list_empty(list)) {
>  		page = lru_to_page(list);
>  		VM_BUG_ON_PAGE(PageLRU(page), page);
> +		list_del(&page->lru);
>  		if (unlikely(!page_evictable(page))) {
> -			list_del(&page->lru);
>  			spin_unlock_irq(&pgdat->lru_lock);
>  			putback_lru_page(page);
>  			spin_lock_irq(&pgdat->lru_lock);
> @@ -1862,9 +1862,7 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
>  		SetPageLRU(page);
>  		lru = page_lru(page);
>  
> -		nr_pages = thp_nr_pages(page);
> -		update_lru_size(lruvec, lru, page_zonenum(page), nr_pages);
> -		list_move(&page->lru, &lruvec->lists[lru]);
> +		add_page_to_lru_list(page, lruvec, lru);
>  
>  		if (put_page_testzero(page)) {
>  			__ClearPageLRU(page);
> @@ -1878,6 +1876,7 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
>  			} else
>  				list_add(&page->lru, &pages_to_free);
>  		} else {
> +			nr_pages = thp_nr_pages(page);
>  			nr_moved += nr_pages;
>  			if (PageActive(page))
>  				workingset_age_nonresident(lruvec, nr_pages);
> -- 
> 2.28.0.681.g6f77f65b4e-goog

-- 
Michal Hocko
SUSE Labs