From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io1-f69.google.com (mail-io1-f69.google.com [209.85.166.69]) by kanga.kvack.org (Postfix) with ESMTP id 2B8DC6B0269 for ; Mon, 8 Oct 2018 05:27:54 -0400 (EDT) Received: by mail-io1-f69.google.com with SMTP id y6-v6so8247374ioc.10 for ; Mon, 08 Oct 2018 02:27:54 -0700 (PDT) Received: from www262.sakura.ne.jp (www262.sakura.ne.jp. [202.181.97.72]) by mx.google.com with ESMTPS id i20-v6si4789408jaf.106.2018.10.08.02.27.52 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 08 Oct 2018 02:27:53 -0700 (PDT) Subject: Re: [PATCH] mm, oom_adj: avoid meaningless loop to find processes sharing mm References: <67eedc4c-7afa-e845-6c88-9716fd820de6@i-love.sakura.ne.jp> <20181008011931epcms1p82dd01b7e5c067ea99946418bc97de46a@epcms1p8> <20181008061407epcms1p519703ae6373a770160c8f912c7aa9521@epcms1p5> <20181008083855epcms1p20e691e5a001f3b94b267997c24e91128@epcms1p2> From: Tetsuo Handa Message-ID: Date: Mon, 8 Oct 2018 18:27:39 +0900 MIME-Version: 1.0 In-Reply-To: <20181008083855epcms1p20e691e5a001f3b94b267997c24e91128@epcms1p2> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: ytk.lee@samsung.com, "mhocko@kernel.org" , "mhocko@suse.com" Cc: "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Oleg Nesterov , David Rientjes , Vladimir Davydov , Andrew Morton , Linus Torvalds On 2018/10/08 17:38, Yong-Taek Lee wrote: >> >> On 2018/10/08 15:14, Yong-Taek Lee wrote: >>>> On 2018/10/08 10:19, Yong-Taek Lee wrote: >>>>> @@ -1056,6 +1056,7 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy) >>>>> struct mm_struct *mm = NULL; >>>>> struct task_struct *task; >>>>> int err = 0; >>>>> + int mm_users = 0; >>>>> >>>>> task = get_proc_task(file_inode(file)); >>>>> if (!task) >>>>> @@ -1092,7 +1093,8 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy) >>>>> struct task_struct *p = find_lock_task_mm(task); >>>>> >>>>> if (p) { >>>>> - if (atomic_read(&p->mm->mm_users) > 1) { >>>>> + mm_users = atomic_read(&p->mm->mm_users); >>>>> + if ((mm_users > 1) && (mm_users != get_nr_threads(p))) { >>>> >>>> How can this work (even before this patch)? When clone(CLONE_VM without CLONE_THREAD/CLONE_SIGHAND) >>>> is requested, copy_process() calls copy_signal() in order to copy sig->oom_score_adj and >>>> sig->oom_score_adj_min before calling copy_mm() in order to increment mm->mm_users, doesn't it? >>>> Then, we will get two different "struct signal_struct" with different oom_score_adj/oom_score_adj_min >>>> but one "struct mm_struct" shared by two thread groups. >>>> >>> >>> Are you talking about race between __set_oom_adj and copy_process? >>> If so, i agree with your opinion. It can not set oom_score_adj properly for copied process if __set_oom_adj >>> check mm_users before copy_process calls copy_mm after copy_signal. Please correct me if i misunderstood anything. >> >> You understand it correctly. >> >> Reversing copy_signal() and copy_mm() is not sufficient either. We need to use a read/write lock >> (read lock for copy_process() and write lock for __set_oom_adj()) in order to make sure that >> the thread created by clone() becomes reachable from for_each_process() path in __set_oom_adj(). >> > > Thank you for your suggestion. But i think it would be better to seperate to 2 issues. How about think these > issues separately because there are no dependency between race issue and my patch. As i already explained, > for_each_process path is meaningless if there is only one thread group with many threads(mm_users > 1 but > no other thread group sharing same mm). Do you have any other idea to avoid meaningless loop ? Yes. I suggest reverting 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"). > >>> >>>>> mm = p->mm; >>>>> atomic_inc(&mm->mm_count); >>>>> } >>> >> >