On Mon 25-06-12 13:21:01, Glauber Costa wrote: > I have an application that does the following: > > * copy the state of all controllers attached to a hierarchy > * replicate it as a child of the current level. > > I would expect writes to the files to mostly succeed, since they > are inheriting sane values from parents. > > But that is not the case for use_hierarchy. If it is set to 0, we > succeed ok. If we're set to 1, the value of the file is automatically > set to 1 in the children, but if userspace tries to write the > very same 1, it will fail. That same situation happens if we > set use_hierarchy, create a child, and then try to write 1 again. > > Now, there is no reason whatsoever for failing to write a value > that is already there. It doesn't even match the comments, that > states: > > /* If parent's use_hierarchy is set, we can't make any modifications > * in the child subtrees... > > since we are not changing anything. > > The following patch tests the new value against the one we're storing, > and automatically return 0 if we're not proposing a change. Fair enough. > > Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx> > CC: Dhaval Giani <dhaval.giani@xxxxxxxxx> > CC: Michal Hocko <mhocko@xxxxxxx> > CC: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> > CC: Johannes Weiner <hannes@xxxxxxxxxxx> One comment bellow... Acked-by: Michal Hocko <mhocko@xxxxxxx> > --- > mm/memcontrol.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index ac35bcc..cccebbc 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -3779,6 +3779,10 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft, > parent_memcg = mem_cgroup_from_cont(parent); > > cgroup_lock(); > + > + if (memcg->use_hierarchy == val) > + goto out; > + Why do you need cgroup_lock to check the value? Even if we have 2 CPUs racing (one trying to set to 0 other to 1 with use_hierarchy==0) then the "set to 0" operation might fail depending on who hits the cgroup_lock first anyway. So while this is correct I think there is not much point to take the global cgroup lock in this case. > /* > * If parent's use_hierarchy is set, we can't make any modifications > * in the child subtrees. If it is unset, then the change can > @@ -3795,6 +3799,8 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft, > retval = -EBUSY; > } else > retval = -EINVAL; > + > +out: > cgroup_unlock(); > > return retval; > -- > 1.7.10.2 > -- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>