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Date:	Mon, 28 Feb 2011 11:18:22 +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>,
	Johannes Weiner <hannes@...xchg.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH] memcg: clean up migration

On Mon, 28 Feb 2011 00:49:25 +0900
Minchan Kim <minchan.kim@...il.com> wrote:

> This patch cleans up unncessary BUG_ON check and confusing
> charge variable.
> 
> That's because 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.
> 
> And mem_cgroup_prepare_migratio return 0 if only it is successful.
> Otherwise, it jumps to unlock label to clean up so BUG_ON(charge)
> isn;t meaningless.
> 
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Balbir Singh <balbir@...ux.vnet.ibm.com>
> Cc: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Signed-off-by: Minchan Kim <minchan.kim@...il.com>

It looks good to me, but I have one minor comment.

> ---
>  mm/memcontrol.c |    1 +
>  mm/migrate.c    |   14 ++++----------
>  2 files changed, 5 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2fc97fc..6832926 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2872,6 +2872,7 @@ static inline int mem_cgroup_move_swap_account(swp_entry_t entry,
>  /*
>   * Before starting migration, account PAGE_SIZE to mem_cgroup that the old
>   * page belongs to.
> + * Return 0 if charge is successful. Otherwise return -errno.
>   */
>  int mem_cgroup_prepare_migration(struct page *page,
>  	struct page *newpage, struct mem_cgroup **ptr, gfp_t gfp_mask)
> diff --git a/mm/migrate.c b/mm/migrate.c
> index eb083a6..737c2e5 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -622,7 +622,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	int *result = NULL;
>  	struct page *newpage = get_new_page(page, private, &result);
>  	int remap_swapcache = 1;
> -	int charge = 0;
>  	struct mem_cgroup *mem;
>  	struct anon_vma *anon_vma = NULL;
>  
> @@ -637,9 +636,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  		if (unlikely(split_huge_page(page)))
>  			goto move_newpage;
>  
> -	/* prepare cgroup just returns 0 or -ENOMEM */
>  	rc = -EAGAIN;
> -
>  	if (!trylock_page(page)) {
>  		if (!force)
>  			goto move_newpage;
> @@ -678,13 +675,11 @@ 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, GFP_KERNEL);
> -	if (charge == -ENOMEM) {
> -		rc = -ENOMEM;
> +	rc = mem_cgroup_prepare_migration(page, newpage, &mem, GFP_KERNEL);
> +	if (rc)
>  		goto unlock;
> -	}
> -	BUG_ON(charge);
>  
> +	rc = -EAGAIN;
>  	if (PageWriteback(page)) {
>  		if (!force || !sync)
>  			goto uncharge;
How about

	if (mem_cgroup_prepare_migration(..)) {
		rc = -ENOMEM;
		goto unlock;
	}

?

Re-setting "rc" to -EAGAIN is not necessary in this case.
"if (mem_cgroup_...)" is commonly used in many places.

Anyway,

	Acked-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>


Thanks,
Daisuke Nishimura.

> @@ -767,8 +762,7 @@ skip_unmap:
>  		drop_anon_vma(anon_vma);
>  
>  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);
>  
> -- 
> 1.7.1
> 
--
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