Re: [PATCH 2/2] mm/memcontrol: add check for allocation failure in mem_cgroup_init()

[Michal Hocko]

On Thu 15-06-23 07:32:26, Haifeng Xu wrote:
> If mem_cgroup_init() fails to allocate mem_cgroup_tree_per_node, we
> should not try to initilaize it. Add check for this case to avoid
> potential NULL pointer dereference.

Technically yes and it seems that all users of soft_limit_tree.rb_tree_per_node
correctly check for NULL so this would be graceful failure handling. At
least superficially because the feature itself would be semi-broken when
used. But more practically this is a 24B allocation and if we fail to
allocate that early during the boot we are screwed anyway. Would such
a system have any chance to boot all the way to userspace? Woul any
userspace actually work?

Is this patch motivated by a code reading or is there any actual
practical upside of handling the error here?
 
> Signed-off-by: Haifeng Xu <haifeng.xu@xxxxxxxxxx>
> ---
>  mm/memcontrol.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c73c5fb33f65..7ebf64e48b25 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -7422,6 +7422,8 @@ static int __init mem_cgroup_init(void)
>  		struct mem_cgroup_tree_per_node *rtpn;
>  
>  		rtpn = kzalloc_node(sizeof(*rtpn), GFP_KERNEL, node);
> +		if (!rtpn)
> +			continue;
>  
>  		rtpn->rb_root = RB_ROOT;
>  		rtpn->rb_rightmost = NULL;
> -- 
> 2.25.1

-- 
Michal Hocko
SUSE Labs