Re: [patch 4/8] mm: memcg: lookup_page_cgroup (almost) never returns NULL

[Michal Hocko <mhocko@suse.cz>]

On Wed 23-11-11 16:42:27, Johannes Weiner wrote:
> From: Johannes Weiner <jweiner@redhat.com>
> 
> Pages have their corresponding page_cgroup descriptors set up before
> they are used in userspace, and thus managed by a memory cgroup.
> 
> The only time where lookup_page_cgroup() can return NULL is in the
> page sanity checking code that executes while feeding pages into the
> page allocator for the first time.
> 
> Remove the NULL checks against lookup_page_cgroup() results from all
> callsites where we know that corresponding page_cgroup descriptors
> must be allocated.

OK, shouldn't we add

diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index 2d123f9..cb93f64 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -35,8 +35,7 @@ struct page_cgroup *lookup_page_cgroup(struct page *page)
 	struct page_cgroup *base;
 
 	base = NODE_DATA(page_to_nid(page))->node_page_cgroup;
-	if (unlikely(!base))
-		return NULL;
+	BUG_ON(!base);
 
 	offset = pfn - NODE_DATA(page_to_nid(page))->node_start_pfn;
 	return base + offset;
@@ -112,8 +111,7 @@ struct page_cgroup *lookup_page_cgroup(struct page *page)
 	unsigned long pfn = page_to_pfn(page);
 	struct mem_section *section = __pfn_to_section(pfn);
 
-	if (!section->page_cgroup)
-		return NULL;
+	BUG_ON(!section->page_cgroup);
 	return section->page_cgroup + pfn;
 }
 
just to make it explicit?

> 
> Signed-off-by: Johannes Weiner <jweiner@redhat.com>

Other than that
Acked-by: Michal Hocko <mhocko@suse.cz>

> ---
>  mm/memcontrol.c |    8 ++------
>  1 files changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d825af9..d4d139a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1894,9 +1894,6 @@ void mem_cgroup_update_page_stat(struct page *page,
>  	bool need_unlock = false;
>  	unsigned long uninitialized_var(flags);
>  
> -	if (unlikely(!pc))
> -		return;
> -
>  	rcu_read_lock();
>  	memcg = pc->mem_cgroup;
>  	if (unlikely(!memcg || !PageCgroupUsed(pc)))
> @@ -2669,8 +2666,6 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
>  	}
>  
>  	pc = lookup_page_cgroup(page);
> -	BUG_ON(!pc); /* XXX: remove this and move pc lookup into commit */
> -
>  	ret = __mem_cgroup_try_charge(mm, gfp_mask, nr_pages, &memcg, oom);
>  	if (ret || !memcg)
>  		return ret;
> @@ -2942,7 +2937,7 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
>  	 * Check if our page_cgroup is valid
>  	 */
>  	pc = lookup_page_cgroup(page);
> -	if (unlikely(!pc || !PageCgroupUsed(pc)))
> +	if (unlikely(!PageCgroupUsed(pc)))
>  		return NULL;
>  
>  	lock_page_cgroup(pc);
> @@ -3326,6 +3321,7 @@ static struct page_cgroup *lookup_page_cgroup_used(struct page *page)
>  	struct page_cgroup *pc;
>  
>  	pc = lookup_page_cgroup(page);
> +	/* Can be NULL while bootstrapping the page allocator */
>  	if (likely(pc) && PageCgroupUsed(pc))
>  		return pc;
>  	return NULL;
> -- 
> 1.7.6.4
> 

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic