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    SubjectRe: [RFC][PATCH 5/4] memcg: don't call res_counter_uncharge when obsolete
    On Thu, 15 Jan 2009 13:38:14 +0900
    Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

    > On Thu, 15 Jan 2009 11:14:20 +0900, Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
    > > > > > To handle the problem "parent may be obsolete",
    > > > > >
    > > > > > call mem_cgroup_get(parent) at create()
    > > > > > call mem_cgroup_put(parent) at freeing memcg.
    > > > > > (regardless of use_hierarchy.)
    > > > > >
    > > > > > is clearer way to go, I think.
    > > > > >
    > > > > > I wonder whether there is mis-accounting problem or not..
    > > > > >
    > hmm, after more consideration, although this patch can prevent the BUG,
    > it can leak memsw accounting of parents because memsw of parents, which
    > have been incremented by charge, does not decremented.
    >
    > I'll try pet/put parent approach..
    > Or any other good ideas ?
    >
    >
    I believe get/put at create/destroy is enough now..
    Let's try and see what happens.

    Thanks,
    -Kame


    > Thanks,
    > Daisuke Nishimura.
    >
    > > > > > So, adding css_tryget() around problematic code can be a fix.
    > > > > > --
    > > > > > mem = swap_cgroup_record();
    > > > > > if (css_tryget(&mem->css)) {
    > > > > > res_counter_uncharge(&mem->memsw, PAZE_SIZE);
    > > > > > css_put(&mem->css)
    > > > > > }
    > > > > > --
    > > > > > I like css_tryget() rather than mem_cgroup_obsolete().
    > > > > I agree.
    > > > > The updated version is attached.
    > > > >
    > > > >
    > > > > Thanks,
    > > > > Daisuke nishimura.
    > > > >
    > > > > > To be honest, I'd like to remove memcg special stuff when I can.
    > > > > >
    > > > > > Thanks,
    > > > > > -Kame
    > > > > >
    > > > > ===
    > > > > From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
    > > > >
    > > > > mem_cgroup_get ensures that the memcg that has been got can be accessed
    > > > > even after the directory has been removed, but it doesn't ensure that parents
    > > > > of it can be accessed: parents might have been freed already by rmdir.
    > > > >
    > > > > This causes a bug in case of use_hierarchy==1, because res_counter_uncharge
    > > > > climb up the tree.
    > > > >
    > > > > Check if the memcg is obsolete by css_tryget, and don't call
    > > > > res_counter_uncharge when obsole.
    > > > >
    > > > > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
    > > > seems nice loock.
    > > >
    > > >
    > > > > ---
    > > > > mm/memcontrol.c | 15 ++++++++++++---
    > > > > 1 files changed, 12 insertions(+), 3 deletions(-)
    > > > >
    > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > > > > index fb62b43..4e3b100 100644
    > > > > --- a/mm/memcontrol.c
    > > > > +++ b/mm/memcontrol.c
    > > > > @@ -1182,7 +1182,10 @@ int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
    > > > > /* avoid double counting */
    > > > > mem = swap_cgroup_record(ent, NULL);
    > > > > if (mem) {
    > > > > - res_counter_uncharge(&mem->memsw, PAGE_SIZE);
    > > > > + if (!css_tryget(&mem->css)) {
    > > > > + res_counter_uncharge(&mem->memsw, PAGE_SIZE);
    > > > > + css_put(&mem->css);
    > > > > + }
    > > > > mem_cgroup_put(mem);
    > > > > }
    > > > > }
    > > >
    > > > I think css_tryget() returns "ture" at success....
    > > >
    > > > So,
    > > > ==
    > > > if (mem && css_tryget(&mem->css))
    > > > res_counter....
    > > >
    > > > is correct.
    > > >
    > > > -Kame
    > > >
    > > Ooops! you are right.
    > > Sorry for my silly mistake..
    > >
    > > "mem" is checked beforehand, so I think css_tryget would be enough.
    > > I'm now testing the attached one.
    > >
    > >
    > > Thanks,
    > > Daisuke Nishimura.
    > > ===
    > > From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
    > >
    > > mem_cgroup_get ensures that the memcg that has been got can be accessed
    > > even after the directory has been removed, but it doesn't ensure that parents
    > > of it can be accessed: parents might have been freed already by rmdir.
    > >
    > > This causes a bug in case of use_hierarchy==1, because res_counter_uncharge
    > > climb up the tree.
    > >
    > > Check if the memcg is obsolete by css_tryget, and don't call
    > > res_counter_uncharge when obsole.
    > >
    > > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
    > > ---
    > > mm/memcontrol.c | 15 ++++++++++++---
    > > 1 files changed, 12 insertions(+), 3 deletions(-)
    > >
    > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > > index fb62b43..b9d5271 100644
    > > --- a/mm/memcontrol.c
    > > +++ b/mm/memcontrol.c
    > > @@ -1182,7 +1182,10 @@ int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
    > > /* avoid double counting */
    > > mem = swap_cgroup_record(ent, NULL);
    > > if (mem) {
    > > - res_counter_uncharge(&mem->memsw, PAGE_SIZE);
    > > + if (css_tryget(&mem->css)) {
    > > + res_counter_uncharge(&mem->memsw, PAGE_SIZE);
    > > + css_put(&mem->css);
    > > + }
    > > mem_cgroup_put(mem);
    > > }
    > > }
    > > @@ -1252,7 +1255,10 @@ void mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr)
    > > struct mem_cgroup *memcg;
    > > memcg = swap_cgroup_record(ent, NULL);
    > > if (memcg) {
    > > - res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
    > > + if (css_tryget(&memcg->css)) {
    > > + res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
    > > + css_put(&memcg->css);
    > > + }
    > > mem_cgroup_put(memcg);
    > > }
    > >
    > > @@ -1397,7 +1403,10 @@ void mem_cgroup_uncharge_swap(swp_entry_t ent)
    > >
    > > memcg = swap_cgroup_record(ent, NULL);
    > > if (memcg) {
    > > - res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
    > > + if (css_tryget(&memcg->css)) {
    > > + res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
    > > + css_put(&memcg->css);
    > > + }
    > > mem_cgroup_put(memcg);
    > > }
    > > }
    >



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