On Wed, Oct 28, 2020 at 11:50 AM Muchun Song <[email protected]> wrote: > > The rcu_read_lock/unlock only can guarantee that the memcg will > not be freed, but it cannot guarantee the success of css_get to > memcg. > > If the whole process of a cgroup offlining is completed between > reading a objcg->memcg pointer and bumping the css reference on > another CPU, and there are exactly 0 external references to this > memory cgroup (how we get to the obj_cgroup_charge() then?), > css_get() can change the ref counter from 0 back to 1. > > Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API") > Signed-off-by: Muchun Song <[email protected]> > Acked-by: Roman Gushchin <[email protected]>
Hi Andrew, Maybe you forgot to add this to the queue for the merge window? Thanks. > --- > changelog in v2: > 1. Add unlikely and update the commit log suggested by Roman. > > mm/memcontrol.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 8c8b4c3ed5a0..d9cdf899c6fc 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -3221,8 +3221,10 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t > gfp, size_t size) > * independently later. > */ > rcu_read_lock(); > +retry: > memcg = obj_cgroup_memcg(objcg); > - css_get(&memcg->css); > + if (unlikely(!css_tryget(&memcg->css))) > + goto retry; > rcu_read_unlock(); > > nr_pages = size >> PAGE_SHIFT; > -- > 2.20.1 > -- Yours, Muchun
