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[195.135.220.15]) by mx.google.com with ESMTPS id j35si3519200pgl.223.2019.01.30.23.11.35 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 30 Jan 2019 23:11:35 -0800 (PST) Received-SPF: softfail (google.com: domain of transitioning mhocko@kernel.org does not designate 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=softfail (google.com: domain of transitioning mhocko@kernel.org does not designate 195.135.220.15 as permitted sender) smtp.mailfrom=mhocko@kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=kernel.org X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id B2B03AF51; Thu, 31 Jan 2019 07:11:32 +0000 (UTC) Date: Thu, 31 Jan 2019 08:11:30 +0100 From: Michal Hocko To: Tetsuo Handa Cc: Andrew Morton , Johannes Weiner , David Rientjes , linux-mm@kvack.org, Yong-Taek Lee , Paul McKenney , Linus Torvalds , LKML Subject: Re: [PATCH v2] mm, oom: Tolerate processes sharing mm with different view of oom_score_adj. Message-ID: <20190131071130.GM18811@dhcp22.suse.cz> References: <1547636121-9229-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> <20190116110937.GI24149@dhcp22.suse.cz> <88e10029-f3d9-5bb5-be46-a3547c54de28@I-love.SAKURA.ne.jp> <20190116121915.GJ24149@dhcp22.suse.cz> <6118fa8a-7344-b4b2-36ce-d77d495fba69@i-love.sakura.ne.jp> <20190116134131.GP24149@dhcp22.suse.cz> <20190117155159.GA4087@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu 31-01-19 07:49:35, Tetsuo Handa wrote: > This patch reverts both commit 44a70adec910d692 ("mm, oom_adj: make sure > processes sharing mm have same view of oom_score_adj") and commit > 97fd49c2355ffded ("mm, oom: kill all tasks sharing the mm") in order to > close a race and reduce the latency at __set_oom_adj(), and reduces the > warning at __oom_kill_process() in order to minimize the latency. > > Commit 36324a990cf578b5 ("oom: clear TIF_MEMDIE after oom_reaper managed > to unmap the address space") introduced the worst case mentioned in > 44a70adec910d692. But since the OOM killer skips mm with MMF_OOM_SKIP set, > only administrators can trigger the worst case. > > Since 44a70adec910d692 did not take latency into account, we can "hold RCU > for minutes and trigger RCU stall warnings" by calling printk() on many > thousands of thread groups. Also, current code becomes a DoS attack vector > which will allow "stalling for more than one month in unkillable state" > simply printk()ing same messages when many thousands of thread groups > tried to iterate __set_oom_adj() on each other. > > I also noticed that 44a70adec910d692 is racy [1], and trying to fix the > race will require a global lock which is too costly for rare events. And > Michal Hocko is thinking to change the oom_score_adj implementation to per > mm_struct (with shadowed score stored in per task_struct in order to > support vfork() => __set_oom_adj() => execve() sequence) so that we don't > need the global lock. > > If the worst case in 44a70adec910d692 happened, it is an administrator's > request. Therefore, before changing the oom_score_adj implementation, > let's eliminate the DoS attack vector first. This is really ridiculous. I have already nacked the previous version and provided two ways around. The simplest one is to drop the printk. The second one is to move oom_score_adj to the mm struct. Could you explain why do you still push for this? > [1] https://lkml.kernel.org/r/20181008011931epcms1p82dd01b7e5c067ea99946418bc97de46a@epcms1p8 > > Signed-off-by: Tetsuo Handa > Reported-by: Yong-Taek Lee > Nacked-by: Michal Hocko > --- > fs/proc/base.c | 46 ---------------------------------------------- > include/linux/mm.h | 2 -- > mm/oom_kill.c | 10 ++++++---- > 3 files changed, 6 insertions(+), 52 deletions(-) > > diff --git a/fs/proc/base.c b/fs/proc/base.c > index 633a634..41ece8f 100644 > --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -1020,7 +1020,6 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count, > static int __set_oom_adj(struct file *file, int oom_adj, bool legacy) > { > static DEFINE_MUTEX(oom_adj_mutex); > - struct mm_struct *mm = NULL; > struct task_struct *task; > int err = 0; > > @@ -1050,55 +1049,10 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy) > } > } > > - /* > - * Make sure we will check other processes sharing the mm if this is > - * not vfrok which wants its own oom_score_adj. > - * pin the mm so it doesn't go away and get reused after task_unlock > - */ > - if (!task->vfork_done) { > - struct task_struct *p = find_lock_task_mm(task); > - > - if (p) { > - if (atomic_read(&p->mm->mm_users) > 1) { > - mm = p->mm; > - mmgrab(mm); > - } > - task_unlock(p); > - } > - } > - > task->signal->oom_score_adj = oom_adj; > if (!legacy && has_capability_noaudit(current, CAP_SYS_RESOURCE)) > task->signal->oom_score_adj_min = (short)oom_adj; > trace_oom_score_adj_update(task); > - > - if (mm) { > - struct task_struct *p; > - > - rcu_read_lock(); > - for_each_process(p) { > - if (same_thread_group(task, p)) > - continue; > - > - /* do not touch kernel threads or the global init */ > - if (p->flags & PF_KTHREAD || is_global_init(p)) > - continue; > - > - task_lock(p); > - if (!p->vfork_done && process_shares_mm(p, mm)) { > - pr_info("updating oom_score_adj for %d (%s) from %d to %d because it shares mm with %d (%s). Report if this is unexpected.\n", > - task_pid_nr(p), p->comm, > - p->signal->oom_score_adj, oom_adj, > - task_pid_nr(task), task->comm); > - p->signal->oom_score_adj = oom_adj; > - if (!legacy && has_capability_noaudit(current, CAP_SYS_RESOURCE)) > - p->signal->oom_score_adj_min = (short)oom_adj; > - } > - task_unlock(p); > - } > - rcu_read_unlock(); > - mmdrop(mm); > - } > err_unlock: > mutex_unlock(&oom_adj_mutex); > put_task_struct(task); > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 80bb640..28879c1 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -2690,8 +2690,6 @@ static inline int in_gate_area(struct mm_struct *mm, unsigned long addr) > } > #endif /* __HAVE_ARCH_GATE_AREA */ > > -extern bool process_shares_mm(struct task_struct *p, struct mm_struct *mm); > - > #ifdef CONFIG_SYSCTL > extern int sysctl_drop_caches; > int drop_caches_sysctl_handler(struct ctl_table *, int, > diff --git a/mm/oom_kill.c b/mm/oom_kill.c > index f0e8cd9..c7005b1 100644 > --- a/mm/oom_kill.c > +++ b/mm/oom_kill.c > @@ -478,7 +478,7 @@ static void dump_header(struct oom_control *oc, struct task_struct *p) > * task's threads: if one of those is using this mm then this task was also > * using it. > */ > -bool process_shares_mm(struct task_struct *p, struct mm_struct *mm) > +static bool process_shares_mm(struct task_struct *p, struct mm_struct *mm) > { > struct task_struct *t; > > @@ -896,12 +896,14 @@ static void __oom_kill_process(struct task_struct *victim) > continue; > if (same_thread_group(p, victim)) > continue; > - if (is_global_init(p)) { > + if (is_global_init(p) || > + p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN) { > can_oom_reap = false; > - set_bit(MMF_OOM_SKIP, &mm->flags); > - pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n", > + if (!test_bit(MMF_OOM_SKIP, &mm->flags)) > + pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n", > task_pid_nr(victim), victim->comm, > task_pid_nr(p), p->comm); > + set_bit(MMF_OOM_SKIP, &mm->flags); > continue; > } > /* > -- > 1.8.3.1 -- Michal Hocko SUSE Labs