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SubjectRe: [PATCH] mm: memcg: keep root group unchanged if fail to create new
On Sun, 11 Dec 2011, Hillf Danton wrote:

> If the request is not to create root group and we fail to meet it,
> we'd leave the root unchanged.

I didn't understand that at first: please say "we should" rather
than "we'd", which I take to be an abbreviation for "we would".

>
> Signed-off-by: Hillf Danton <dhillf@gmail.com>

Yes indeed, well caught:
Acked-by: Hugh Dickins <hughd@google.com>

I wonder what was going through the author's mind when he wrote it
that way? I wonder if it's one of those bugs that creeps in when
you start from a perfectly functional patch, then make refinements
to suit feedback from reviewers.

On which topic: wouldn't this patch be better just to move the
"root_mem_cgroup = memcg;" two lines lower down (and of course
remove free_out's "root_mem_cgroup = NULL;" as you already did)?
I can't see mem_cgroup_soft_limit_tree_init() relying on
root_mem_cgroup at all.

Should your patch go to stable, even to 2.6.32-longterm?
Matter of taste, really: while it's not quite impossible for
alloc_mem_cgroup_per_zone_info() to fail, it is very unlikely.

> ---
>
> --- a/mm/memcontrol.c Fri Dec 9 21:57:40 2011
> +++ b/mm/memcontrol.c Sun Dec 11 09:04:48 2011
> @@ -4849,8 +4849,10 @@ mem_cgroup_create(struct cgroup_subsys *
> enable_swap_cgroup();
> parent = NULL;
> root_mem_cgroup = memcg;
> - if (mem_cgroup_soft_limit_tree_init())
> + if (mem_cgroup_soft_limit_tree_init()) {
> + root_mem_cgroup = NULL;
> goto free_out;
> + }
> for_each_possible_cpu(cpu) {
> struct memcg_stock_pcp *stock =
> &per_cpu(memcg_stock, cpu);
> @@ -4888,7 +4890,6 @@ mem_cgroup_create(struct cgroup_subsys *
> return &memcg->css;
> free_out:
> __mem_cgroup_free(memcg);
> - root_mem_cgroup = NULL;
> return ERR_PTR(error);
> }
>


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