Re: [BUGFIX][PATCH mmotm] memcg: try_get_mem_cgroup_from_swapcache fix

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* Daisuke Nishimura <d-nishimura@mtf.biglobe.ne.jp> [2009-03-23 00:02:38]:

> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> 
> css_tryget can be called twice in !PageCgroupUsed case.
> 
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> ---
> This is a fix for cgroups-use-css-id-in-swap-cgroup-for-saving-memory-v5.patch
> 
>  mm/memcontrol.c |   10 ++++------
>  1 files changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5de6be9..55dea59 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1027,9 +1027,11 @@ static struct mem_cgroup *try_get_mem_cgroup_from_swapcache(struct page *page)
>  	/*
>  	 * Used bit of swapcache is solid under page lock.
>  	 */
> -	if (PageCgroupUsed(pc))
> +	if (PageCgroupUsed(pc)) {
>  		mem = pc->mem_cgroup;
> -	else {
> +		if (mem && !css_tryget(&mem->css))
> +			mem = NULL;
> +	} else {
>  		ent.val = page_private(page);
>  		id = lookup_swap_cgroup(ent);
>  		rcu_read_lock();
> @@ -1038,10 +1040,6 @@ static struct mem_cgroup *try_get_mem_cgroup_from_swapcache(struct page *page)
>  			mem = NULL;
>  		rcu_read_unlock();
>  	}
> -	if (!mem)
> -		return NULL;
> -	if (!css_tryget(&mem->css))
> -		return NULL;
>  	return mem;
>  }

How did you detect the problem? Any test case/steps to reproduce the issue?


-- 
	Balbir

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