linux-kernel - Re: [PATCH] mm/swap: Use page flags to determine LRU list in __activate

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Message-ID: <20171019153322.c4uqalws7l7fdzcx@dhcp22.suse.cz>
Date:   Thu, 19 Oct 2017 17:33:22 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, shli@...nel.org
Subject: Re: [PATCH] mm/swap: Use page flags to determine LRU list in
 __activate_page()

On Thu 19-10-17 20:26:57, Anshuman Khandual wrote:
> Its already assumed that the PageActive flag is clear on the input
> page, hence page_lru(page) will pick the base LRU for the page. In
> the same way page_lru(page) will pick active base LRU, once the
> flag PageActive is set on the page. This change of LRU list should
> happen implicitly through the page flags instead of being hard
> coded.

The patch description tells what but it doesn't explain _why_? Does the
resulting code is better, more optimized or is this a pure readability
thing?

All I can see is that page_lru is more complex and a large part of it
can be optimized away which has been done manually here. I suspect the
compiler can deduce the same thing.

> Signed-off-by: Anshuman Khandual <khandual@...ux.vnet.ibm.com>
> ---
>  mm/swap.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index fcd82bc..494276b 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -275,12 +275,10 @@ static void __activate_page(struct page *page, struct lruvec *lruvec,
>  {
>  	if (PageLRU(page) && !PageActive(page) && !PageUnevictable(page)) {
>  		int file = page_is_file_cache(page);
> -		int lru = page_lru_base_type(page);
>  
> -		del_page_from_lru_list(page, lruvec, lru);
> +		del_page_from_lru_list(page, lruvec, page_lru(page));
>  		SetPageActive(page);
> -		lru += LRU_ACTIVE;
> -		add_page_to_lru_list(page, lruvec, lru);
> +		add_page_to_lru_list(page, lruvec, page_lru(page));
>  		trace_mm_lru_activate(page);
>  
>  		__count_vm_event(PGACTIVATE);
> -- 
> 1.8.5.2

-- 
Michal Hocko
SUSE Labs