[patch rfc] memcg: remove NULL check from lookup_page_cgroup() result

[patch rfc] memcg: remove NULL check from lookup_page_cgroup() result

[Johannes Weiner]

The page_cgroup array is set up before even fork is initialized.  I
seriously doubt that this code executes before the array is alloc'd.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/memcontrol.c |    5 +----
 1 files changed, 1 insertions(+), 4 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a145c9e..6abaa10 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2343,10 +2343,7 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
 	}
 
 	pc = lookup_page_cgroup(page);
-	/* can happen at boot */
-	if (unlikely(!pc))
-		return 0;
-	prefetchw(pc);
+	BUG_ON(!pc); /* XXX: remove this and move pc lookup into commit */
 
 	ret = __mem_cgroup_try_charge(mm, gfp_mask, &mem, oom, page_size);
 	if (ret || !mem)
-- 
1.7.4

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Re: [patch rfc] memcg: remove NULL check from lookup_page_cgroup() result

[KAMEZAWA Hiroyuki]

On Thu, 3 Feb 2011 15:12:30 +0100
Johannes Weiner <hannes@cmpxchg.org> wrote:

> The page_cgroup array is set up before even fork is initialized.  I
> seriously doubt that this code executes before the array is alloc'd.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

I don't have solid answer to this. If some module use radix-tree and enter pages
to it, mem_cgroup may see it. But..

For my opinion, tring this is good.

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

> ---
>  mm/memcontrol.c |    5 +----
>  1 files changed, 1 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index a145c9e..6abaa10 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2343,10 +2343,7 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
>  	}
>  
>  	pc = lookup_page_cgroup(page);
> -	/* can happen at boot */
> -	if (unlikely(!pc))
> -		return 0;
> -	prefetchw(pc);
> +	BUG_ON(!pc); /* XXX: remove this and move pc lookup into commit */
>  
>  	ret = __mem_cgroup_try_charge(mm, gfp_mask, &mem, oom, page_size);
>  	if (ret || !mem)
> -- 
> 1.7.4
> 

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