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SubjectRe: [RFC][PATCH 1/2] memcg: Ensure every task that uses an mm is in the same memory cgroup
On Fri 01-06-18 09:53:09, Eric W. Biederman wrote:
[...]
> +static int mem_cgroup_mm_can_attach(struct cgroup_taskset *tset)
> +{
> + struct cgroup_subsys_state *css, *tcss;
> + struct task_struct *p, *t;
> + struct mm_struct *mm = NULL;
> + int ret = -EINVAL;
> +
> + /*
> + * Ensure all references for every mm can be accounted for by
> + * tasks that are being migrated.
> + */
> + rcu_read_lock();
> + cgroup_taskset_for_each(p, css, tset) {
> + int mm_users, mm_count;
> +
> + /* Does this task have an mm that has not been visited? */
> + if (!p->mm || (p->flags & PF_KTHREAD) || (p->mm == mm))
> + continue;
> +
> + mm = p->mm;
> + mm_users = atomic_read(&mm->mm_users);
> + if (mm_users == 1)
> + continue;
> +
> + mm_count = 0;
> + cgroup_taskset_for_each(t, tcss, tset) {
> + if (t->mm != mm)
> + continue;
> + mm_count++;
> + }
> + /*
> + * If there are no non-task references mm_users will
> + * be stable as holding cgroup_thread_rwsem for write
> + * blocks fork and exit.
> + */
> + if (mm_count != mm_users)
> + goto out;

Wouldn't it be much more simple to do something like this instead? Sure
it will break migration of non-thread tasks sharing mms but I would like
to see that this actually matters to anybody rather than make the code
more complicated and maintain it forever without a good reason. So yes
this is a bit harsh but considering that the migration suceeded silently
and that was simply broken as well, I am not sure this is any worse.

Btw. MMF_ALIEN_MM could be used in the OOM proper as well.

diff --git a/kernel/fork.c b/kernel/fork.c
index dfe8e14c0fba..285cd0397307 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1303,6 +1303,8 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
if (clone_flags & CLONE_VM) {
mmget(oldmm);
mm = oldmm;
+ if (unlikely(!(clone_flags & CLONE_THREAD)))
+ set_bit(MMF_ALIEN_MM, &mm->flags);
goto good_mm;
}

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2f5dac193431..fa0248fcdb36 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5103,6 +5103,18 @@ static int mem_cgroup_can_attach(struct cgroup_taskset *tset)
if (!mm)
return 0;

+ /*
+ * Migrating a task which shares mm with other thread group
+ * has never been really well defined. Shout to the log when
+ * this happens and see whether anybody really complains.
+ * If so make sure to support migration if all processes sharing
+ * this mm are migrating together.
+ */
+ if (WARN_ON_ONCE(test_bit(MMF_ALIEN_MM, &mm->flags))) {
+ mmput(mm);
+ return -EBUSY;
+ }
+
/* We move charges except for creative uses of CLONE_VM */
if (mm->memcg == from) {
VM_BUG_ON(mc.from);
--
Michal Hocko
SUSE Labs

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