On 2018/09/06 20:53, Michal Hocko wrote: > On Thu 06-09-18 20:40:34, Tetsuo Handa wrote: >> On 2018/09/06 20:23, Michal Hocko wrote: >>> On Thu 06-09-18 19:58:25, Tetsuo Handa wrote: >>> [...] >>>> >From 18876f287dd69a7c33f65c91cfcda3564233f55e Mon Sep 17 00:00:00 2001 >>>> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> >>>> Date: Thu, 6 Sep 2018 19:53:18 +0900 >>>> Subject: [PATCH] mm, oom: Introduce time limit for dump_tasks duration. >>>> >>>> Since printk() is slow, printing one line takes nearly 0.01 second. >>>> As a result, syzbot is stalling for 52 seconds trying to dump 5600 >>>> tasks at for_each_process() under RCU. Since such situation is almost >>>> inflight fork bomb attack (the OOM killer will print similar tasks for >>>> so many times), it makes little sense to print all candidate tasks. >>>> Thus, this patch introduces 3 seconds limit for printing. >>>> >>>> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> >>>> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx> >>> >>> You really love timeout based solutions with randomly chosen timeouts, >>> don't you. This is just ugly as hell. We already have means to disable >>> tasks dumping (see /proc/sys/vm/oom_dump_tasks). >> >> I know /proc/sys/vm/oom_dump_tasks . Showing some entries while not always >> printing all entries might be helpful. > > Not really. It could be more confusing than helpful. The main purpose of > the listing is to double check the list to understand the oom victim > selection. If you have a partial list you simply cannot do that. It serves as a safeguard for avoiding RCU stall warnings. > > If the iteration takes too long and I can imagine it does with zillions > of tasks then the proper way around it is either release the lock > periodically after N tasks is processed or outright skip the whole thing > if there are too many tasks. The first option is obviously tricky to > prevent from duplicate entries or other artifacts. > Can we add rcu_lock_break() like check_hung_uninterruptible_tasks() does?
