linux-kernel - Re: [PATCH] mm: move swap-in anonymous page into active list

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Message-ID: <20160729133033.GA2034@cmpxchg.org>
Date:	Fri, 29 Jul 2016 09:30:33 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] mm: move swap-in anonymous page into active list

On Fri, Jul 29, 2016 at 12:25:40PM +0900, Minchan Kim wrote:
> Every swap-in anonymous page starts from inactive lru list's head.
> It should be activated unconditionally when VM decide to reclaim
> because page table entry for the page always usually has marked
> accessed bit. Thus, their window size for getting a new referece
> is 2 * NR_inactive + NR_active while others is NR_active + NR_active.
> 
> It's not fair that it has more chance to be referenced compared
> to other newly allocated page which starts from active lru list's
> head.
> 
> Signed-off-by: Minchan Kim <minchan@...nel.org>

That behavior stood out to me as well recently, but I couldn't
convince myself that activation is the right thing.

The page can still have a valid copy on the swap device, so prefering
to reclaim that page over a fresh one could make sense. But as you
point out, having it start inactive instead of active actually ends up
giving it *more* LRU time, and that seems to be without justification.

So this change makes sense to me. Maybe somebody else remembers a good
reason for why the behavior is the way it is, but likely it has always
been an oversight.

Acked-by: Johannes Weiner <hannes@...xchg.org>

> ---
>  mm/memory.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 4425b6059339..3a730b920242 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2642,6 +2642,7 @@ int do_swap_page(struct fault_env *fe, pte_t orig_pte)
>  	if (page == swapcache) {
>  		do_page_add_anon_rmap(page, vma, fe->address, exclusive);
>  		mem_cgroup_commit_charge(page, memcg, true, false);
> +		activate_page(page);
>  	} else { /* ksm created a completely new copy */
>  		page_add_new_anon_rmap(page, vma, fe->address, false);
>  		mem_cgroup_commit_charge(page, memcg, false, false);
> -- 
> 1.9.1