linux-kernel - Re: [PATCH] memcg: remove charge variable in unmap_and

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Date:	Tue, 11 Jan 2011 15:35:13 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH] memcg: remove charge variable in unmap_and_move

Hi,

On Tue, 11 Jan 2011 15:00:50 +0900
Minchan Kim <minchan.kim@...il.com> wrote:

> memcg charge/uncharge could be handled by mem_cgroup_[prepare/end]
> migration itself so charge local variable in unmap_and_move lost the role
> since we introduced 01b1ae63c2.
> 
> In addition, the variable name is not good like below.
> 
> int unmap_and_move()
> {
> 	charge = mem_cgroup_prepare_migration(xxx);
> 	..
> 		BUG_ON(charge); <-- BUG if it is charged?
> 		..
> 		uncharge:
> 		if (!charge)    <-- why do we have to uncharge !charge?
> 			mem_group_end_migration(xxx);
> 	..
> }
> 
> So let's remove unnecessary and confusing variable.
> 
> Suggested-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> Signed-off-by: Minchan Kim <minchan.kim@...il.com>
> Cc: Balbir Singh <balbir@...ux.vnet.ibm.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
>  mm/migrate.c |   12 ++++--------
>  1 files changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index b8a32da..e393841 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -623,7 +623,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	struct page *newpage = get_new_page(page, private, &result);
>  	int remap_swapcache = 1;
>  	int rcu_locked = 0;
> -	int charge = 0;
>  	struct mem_cgroup *mem = NULL;
>  	struct anon_vma *anon_vma = NULL;
>  
> @@ -662,12 +661,10 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	}
>  
>  	/* charge against new page */
> -	charge = mem_cgroup_prepare_migration(page, newpage, &mem);
> -	if (charge == -ENOMEM) {
> -		rc = -ENOMEM;
> +	rc = mem_cgroup_prepare_migration(page, newpage, &mem);
> +	if (rc == -ENOMEM)
>  		goto unlock;
> -	}
> -	BUG_ON(charge);
> +	BUG_ON(rc);
>  
>  	if (PageWriteback(page)) {
>  		if (!force || !sync)
> @@ -760,8 +757,7 @@ rcu_unlock:
>  	if (rcu_locked)
>  		rcu_read_unlock();
>  uncharge:
> -	if (!charge)
> -		mem_cgroup_end_migration(mem, page, newpage, rc == 0);
> +	mem_cgroup_end_migration(mem, page, newpage, rc == 0);
>  unlock:
>  	unlock_page(page);
>  
I proposed pseud code like above, but it's wrong unfortunately.
If mem_cgroup_prepare_migration() has succeeded, rc is overwritten to 0.
So even if we failed before calling move_to_new_page(), rc is 0 and
mem_cgroup_end_migration() mis-understand this migration has succeeded.

And, it seems to be just a bit off-topic, the place of the comment
"prepare cgroup just returns 0 or -ENOMEM" isn't good, seeing the commit e8589cc1,
which introduced the comment first.

So, we should do like:

	/* charge against new page */
	if (mem_cgroup_end_migration(page, &newpage, &mem)) {
		/* prepare_migration just returns 0 or -ENOMEM */
		rc = -ENOMEM;
		goto unlock;
	}

	if (PageWriteback(page)) {
		...

uncharge:
	mem_cgroup_end_migration(mem, page, newpage, rc == 0);

or, overwrite rc to -EAGAIN again.
I don't stick to checking "BUG_ON(charge)" personally.


Thanks,
Daisuke Nishimura.
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