memcg: fix wrong VM_BUG_ON() in try_charge()'s mm->owner check
At __mem_cgroup_try_charge(), VM_BUG_ON(!mm->owner) is checked. But as commented in mem_cgroup_from_task(), mm->owner can be NULL in some racy case. This check of VM_BUG_ON() is bad. A possible story to hit this is at swapoff()->try_to_unuse(). It passes mm_struct to mem_cgroup_try_charge_swapin() while mm->owner is NULL. If we can't get proper mem_cgroup from swap_cgroup information, mm->owner is used as charge target and we see NULL. Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Reported-by: Hugh Dickins <hughd@google.com> Reported-by: Thomas Meyer <thomas@m3y3r.de> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Reviewed-by: Balbir Singh <balbir@linux.vnet.ibm.com> Signed-off-by: Hugh Dickins <hughd@google.com> Cc: stable@kernel.org Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -1925,19 +1925,18 @@ again:
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rcu_read_lock();
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p = rcu_dereference(mm->owner);
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VM_BUG_ON(!p);
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/*
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* because we don't have task_lock(), "p" can exit while
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* we're here. In that case, "mem" can point to root
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* cgroup but never be NULL. (and task_struct itself is freed
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* by RCU, cgroup itself is RCU safe.) Then, we have small
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* risk here to get wrong cgroup. But such kind of mis-account
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* by race always happens because we don't have cgroup_mutex().
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* It's overkill and we allow that small race, here.
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* Because we don't have task_lock(), "p" can exit.
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* In that case, "mem" can point to root or p can be NULL with
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* race with swapoff. Then, we have small risk of mis-accouning.
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* But such kind of mis-account by race always happens because
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* we don't have cgroup_mutex(). It's overkill and we allo that
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* small race, here.
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* (*) swapoff at el will charge against mm-struct not against
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* task-struct. So, mm->owner can be NULL.
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*/
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mem = mem_cgroup_from_task(p);
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VM_BUG_ON(!mem);
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if (mem_cgroup_is_root(mem)) {
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if (!mem || mem_cgroup_is_root(mem)) {
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rcu_read_unlock();
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goto done;
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}
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