memcg: root_cgroup cannot reach mem_cgroup_move_parent
The root cgroup cannot be destroyed so we never hit it down the mem_cgroup_pre_destroy path and mem_cgroup_force_empty_write shouldn't even try to do anything if called for the root. This means that mem_cgroup_move_parent doesn't have to bother with the root cgroup and it can assume it can always move charges upwards. Signed-off-by: Michal Hocko <mhocko@suse.cz> Reviewed-by: Tejun Heo <tj@kernel.org> Reviewed-by: Glauber Costa <glommer@parallels.com> Signed-off-by: Tejun Heo <tj@kernel.org>
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@ -2715,9 +2715,7 @@ static int mem_cgroup_move_parent(struct page *page,
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unsigned long uninitialized_var(flags);
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unsigned long uninitialized_var(flags);
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int ret;
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int ret;
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/* Is ROOT ? */
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VM_BUG_ON(mem_cgroup_is_root(child));
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if (mem_cgroup_is_root(child))
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return -EINVAL;
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ret = -EBUSY;
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ret = -EBUSY;
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if (!get_page_unless_zero(page))
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if (!get_page_unless_zero(page))
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@ -3823,6 +3821,8 @@ static int mem_cgroup_force_empty_write(struct cgroup *cont, unsigned int event)
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struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
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struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
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int ret;
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int ret;
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if (mem_cgroup_is_root(memcg))
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return -EINVAL;
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css_get(&memcg->css);
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css_get(&memcg->css);
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ret = mem_cgroup_force_empty(memcg);
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ret = mem_cgroup_force_empty(memcg);
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css_put(&memcg->css);
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css_put(&memcg->css);
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