linux-kernel - Re: [PATCH 6/9] ksm: mem cgroup charge swapin copy

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Date:	Mon, 30 Nov 2009 09:13:16 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Izik Eidus <ieidus@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Chris Wright <chrisw@...hat.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 6/9] ksm: mem cgroup charge swapin copy

On Tue, 24 Nov 2009 16:51:13 +0000 (GMT)
Hugh Dickins <hugh.dickins@...cali.co.uk> wrote:

> But ksm swapping does require one small change in mem cgroup handling.
> When do_swap_page()'s call to ksm_might_need_to_copy() does indeed
> substitute a duplicate page to accommodate a different anon_vma (or a
> different index), that page escaped mem cgroup accounting, because of
> the !PageSwapCache check in mem_cgroup_try_charge_swapin().
> 
> That was returning success without charging, on the assumption that
> pte_same() would fail after, which is not the case here.  Originally I
> proposed that success, so that an unshrinkable mem cgroup at its limit
> would not fail unnecessarily; but that's a minor point, and there are
> plenty of other places where we may fail an overallocation which might
> later prove unnecessary.  So just go ahead and do what all the other
> exceptions do: proceed to charge current mm.
> 
> Signed-off-by: Hugh Dickins <hugh.dickins@...cali.co.uk>

Ok. Maybe commit_charge will work enough. (I hope so.)

Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

BTW, I'm happy if you adds "How to test" documenation to
Documentation/vm/ksm.txt or to share some test programs.

1. Map anonymous pages + madvise(MADV_MERGEABLE)
2. "echo 1 > /sys/kernel/mm/ksm/run"

is enough ?

Thanks,
-Kame

> ---
> 
>  mm/memcontrol.c |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> --- ksm5/mm/memcontrol.c	2009-11-14 10:17:02.000000000 +0000
> +++ ksm6/mm/memcontrol.c	2009-11-22 20:40:37.000000000 +0000
> @@ -1862,11 +1862,12 @@ int mem_cgroup_try_charge_swapin(struct
>  		goto charge_cur_mm;
>  	/*
>  	 * A racing thread's fault, or swapoff, may have already updated
> -	 * the pte, and even removed page from swap cache: return success
> -	 * to go on to do_swap_page()'s pte_same() test, which should fail.
> +	 * the pte, and even removed page from swap cache: in those cases
> +	 * do_swap_page()'s pte_same() test will fail; but there's also a
> +	 * KSM case which does need to charge the page.
>  	 */
>  	if (!PageSwapCache(page))
> -		return 0;
> +		goto charge_cur_mm;
>  	mem = try_get_mem_cgroup_from_swapcache(page);
>  	if (!mem)
>  		goto charge_cur_mm;
> --
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