From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f200.google.com (mail-pf1-f200.google.com [209.85.210.200]) by kanga.kvack.org (Postfix) with ESMTP id BA9B46B000A for ; Mon, 8 Oct 2018 02:22:43 -0400 (EDT) Received: by mail-pf1-f200.google.com with SMTP id 87-v6so16151227pfq.8 for ; Sun, 07 Oct 2018 23:22:43 -0700 (PDT) Received: from www262.sakura.ne.jp (www262.sakura.ne.jp. [202.181.97.72]) by mx.google.com with ESMTPS id 28-v6si14531923pgy.68.2018.10.07.23.22.42 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 07 Oct 2018 23:22:42 -0700 (PDT) Subject: Re: [PATCH] mm, oom_adj: avoid meaningless loop to find processes sharing mm References: <20181008011931epcms1p82dd01b7e5c067ea99946418bc97de46a@epcms1p8> <20181008061407epcms1p519703ae6373a770160c8f912c7aa9521@epcms1p5> From: Tetsuo Handa Message-ID: <67eedc4c-7afa-e845-6c88-9716fd820de6@i-love.sakura.ne.jp> Date: Mon, 8 Oct 2018 15:22:37 +0900 MIME-Version: 1.0 In-Reply-To: <20181008061407epcms1p519703ae6373a770160c8f912c7aa9521@epcms1p5> 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" 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(). > >>> mm = p->mm; >>> atomic_inc(&mm->mm_count); >>> } >