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    SubjectRe: [PATCH] memcg: use ID in page_cgroup
    On Mon, 23 Aug 2010 14:32:37 +0900
    Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

    > 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".
    >
    I'll use "flags" to remove struct page pointer.


    > > @@ -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 ?
    >

    yes. that's just for rcu_deerference_check().


    > > +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().
    >

    Why ??

    Thanks,
    -Kame





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