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    SubjectRe: [PATCH] memcg: use ID in page_cgroup
    On Fri, 20 Aug 2010 19:01:32 +0900
    KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

    >
    > I have an idea to remove page_cgroup->page pointer, 8bytes reduction per page.
    > But it will be after this work.
    Another off topic. I think we can reduce the size of mem_cgroup by packing
    some boolean members into one "unsinged long flags".

    > @@ -300,12 +300,13 @@ static atomic_t mem_cgroup_num;
    > #define NR_MEMCG_GROUPS (CONFIG_MEM_CGROUP_MAX_GROUPS + 1)
    > static struct mem_cgroup *mem_cgroups[NR_MEMCG_GROUPS] __read_mostly;
    >
    > -/* Must be called under rcu_read_lock */
    > -static struct mem_cgroup *id_to_memcg(unsigned short id)
    > +/* Must be called under rcu_read_lock, set safe==true if under lock */
    Do you mean, "Set safe==true if we can ensure by some locks that the id can be
    safely dereferenced without rcu_read_lock", right ?

    > +static struct mem_cgroup *id_to_memcg(unsigned short id, bool safe)
    > {
    > struct mem_cgroup *ret;
    > /* see mem_cgroup_free() */
    > - ret = rcu_dereference_check(mem_cgroups[id], rch_read_lock_held());
    > + ret = rcu_dereference_check(mem_cgroups[id],
    > + rch_read_lock_held() || safe);
    > if (likely(ret && ret->valid))
    > return ret;
    > return NULL;

    (snip)
    > @@ -723,6 +729,11 @@ static inline bool mem_cgroup_is_root(st
    > return (mem == root_mem_cgroup);
    > }
    >
    > +static inline bool mem_cgroup_is_rootid(unsigned short id)
    > +{
    > + return (id == 1);
    > +}
    > +
    It might be better to add

    BUG_ON(newid->id != 1)

    in cgroup.c::cgroup_init_idr().


    Thanks,
    Daisuke Nishimura.


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