Re: [patch 5/7] mm: page_cgroup: check page_cgroup arrays in lookup_page_cgroup() only when necessary

[Michal Hocko <mhocko@suse.cz>]

On Tue 29-11-11 11:52:03, Johannes Weiner wrote:
> lookup_page_cgroup() is usually used only against pages that are used
> in userspace.
> 
> The exception is the CONFIG_DEBUG_VM-only memcg check from the page
> allocator: it can run on pages without page_cgroup descriptors
> allocated when the pages are fed into the page allocator for the first
> time during boot or memory hotplug.
> 
> Include the array check only when CONFIG_DEBUG_VM is set and save the
> unnecessary check in production kernels.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

I was thinking about adding BUG_ON before dereferencing but this
is questionable because NULL ptr dereference will provide the same
information except sec. guys might be alerted.

I like the smaller code more of course.

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

> ---
>  mm/page_cgroup.c |   18 ++++++++++++++++--
>  1 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
> index a14655d..58405ca 100644
> --- a/mm/page_cgroup.c
> +++ b/mm/page_cgroup.c
> @@ -28,9 +28,16 @@ struct page_cgroup *lookup_page_cgroup(struct page *page)
>  	struct page_cgroup *base;
>  
>  	base = NODE_DATA(page_to_nid(page))->node_page_cgroup;
> +#ifdef CONFIG_DEBUG_VM
> +	/*
> +	 * The sanity checks the page allocator does upon freeing a
> +	 * page can reach here before the page_cgroup arrays are
> +	 * allocated when feeding a range of pages to the allocator
> +	 * for the first time during bootup or memory hotplug.
> +	 */
>  	if (unlikely(!base))
>  		return NULL;
> -
> +#endif
>  	offset = pfn - NODE_DATA(page_to_nid(page))->node_start_pfn;
>  	return base + offset;
>  }
> @@ -87,9 +94,16 @@ struct page_cgroup *lookup_page_cgroup(struct page *page)
>  {
>  	unsigned long pfn = page_to_pfn(page);
>  	struct mem_section *section = __pfn_to_section(pfn);
> -
> +#ifdef CONFIG_DEBUG_VM
> +	/*
> +	 * The sanity checks the page allocator does upon freeing a
> +	 * page can reach here before the page_cgroup arrays are
> +	 * allocated when feeding a range of pages to the allocator
> +	 * for the first time during bootup or memory hotplug.
> +	 */
>  	if (!section->page_cgroup)
>  		return NULL;
> +#endif
>  	return section->page_cgroup + pfn;
>  }
>  
> -- 
> 1.7.6.4
> 

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

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