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    SubjectRe: [BUGFIX][PATCH] memcg: get/put parents at create/free
    On Fri, 16 Jan 2009 10:50:09 +0900 Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

    > This version works well in my test.
    >
    > Andrew, please pick up this one.
    >
    > ===
    > From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
    >
    > The lifetime of struct cgroup and struct mem_cgroup is different and
    > mem_cgroup has its own reference count for handling references from swap_cgroup.
    >
    > This causes strange problem that the parent mem_cgroup dies while
    > child mem_cgroup alive, and this problem causes a bug in case of use_hierarchy==1
    > because res_counter_uncharge climbs up the tree.
    >
    > This patch is for avoiding it by getting the parent at create, and
    > putting it at freeing.
    >
    > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
    > Reviewed-by; KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > ---
    > mm/memcontrol.c | 23 ++++++++++++++++++++++-
    > 1 files changed, 22 insertions(+), 1 deletions(-)
    >
    > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > index fb62b43..45e1b51 100644
    > --- a/mm/memcontrol.c
    > +++ b/mm/memcontrol.c
    > @@ -202,6 +202,8 @@ pcg_default_flags[NR_CHARGE_TYPE] = {
    >
    > static void mem_cgroup_get(struct mem_cgroup *mem);
    > static void mem_cgroup_put(struct mem_cgroup *mem);
    > +static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem);
    > +static void mem_cgroup_get_parent(struct mem_cgroup *mem);
    >
    > static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
    > struct page_cgroup *pc,
    > @@ -2185,10 +2187,28 @@ static void mem_cgroup_get(struct mem_cgroup *mem)
    >
    > static void mem_cgroup_put(struct mem_cgroup *mem)
    > {
    > - if (atomic_dec_and_test(&mem->refcnt))
    > + if (atomic_dec_and_test(&mem->refcnt)) {
    > + struct mem_cgroup *parent = parent_mem_cgroup(mem);
    > __mem_cgroup_free(mem);
    > + if (parent)
    > + mem_cgroup_put(parent);
    > + }
    > +}
    > +
    > +static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem)
    > +{
    > + if (!mem->res.parent)
    > + return NULL;
    > + return mem_cgroup_from_res_counter(mem->res.parent, res);
    > }
    >
    > +static void mem_cgroup_get_parent(struct mem_cgroup *mem)
    > +{
    > + struct mem_cgroup *parent = parent_mem_cgroup(mem);
    > +
    > + if (parent)
    > + mem_cgroup_get(parent);
    > +}
    >
    > #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
    > static void __init enable_swap_cgroup(void)
    > @@ -2237,6 +2257,7 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
    > if (parent)
    > mem->swappiness = get_swappiness(parent);
    > atomic_set(&mem->refcnt, 1);
    > + mem_cgroup_get_parent(mem);
    > return &mem->css;
    > free_out:
    > __mem_cgroup_free(mem);

    It seems strange that we add a little helper function for the get(),
    but open-code the put()?



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