From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f197.google.com (mail-pf0-f197.google.com [209.85.192.197]) by kanga.kvack.org (Postfix) with ESMTP id 424CD6B0005 for ; Fri, 9 Mar 2018 06:42:12 -0500 (EST) Received: by mail-pf0-f197.google.com with SMTP id s25so1540139pfh.9 for ; Fri, 09 Mar 2018 03:42:12 -0800 (PST) Received: from www262.sakura.ne.jp (www262.sakura.ne.jp. [202.181.97.72]) by mx.google.com with ESMTPS id g6si612896pgu.737.2018.03.09.03.42.10 for (version=TLS1 cipher=AES128-SHA bits=128/128); Fri, 09 Mar 2018 03:42:11 -0800 (PST) Subject: Re: [PATCH] mm: oom: Fix race condition between oom_badness and do_exit of task From: Tetsuo Handa References: <1520427454-22813-1-git-send-email-gkohli@codeaurora.org> <22ebd655-ece4-37e5-5a98-e9750cb20665@codeaurora.org> <14ba6c44-d444-bd0a-0bac-0c6851b19344@codeaurora.org> In-Reply-To: <14ba6c44-d444-bd0a-0bac-0c6851b19344@codeaurora.org> Message-Id: <201803091948.FBC21396.LHOMSFFOVFtQJO@I-love.SAKURA.ne.jp> Date: Fri, 9 Mar 2018 19:48:46 +0900 Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: owner-linux-mm@kvack.org List-ID: To: gkohli@codeaurora.org, rientjes@google.com Cc: akpm@linux-foundation.org, mhocko@suse.com, kirill.shutemov@linux.intel.com, aarcange@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org Kohli, Gaurav wrote: > > t->alloc_lock is still held when leaving find_lock_task_mm(), which means > > that t->mm != NULL. But nothing prevents t from setting t->mm = NULL at > > exit_mm() from do_exit() and calling exit_creds() from __put_task_struct(t) > > after task_unlock(t) is called. Seems difficult to trigger race window. Maybe > > something has preempted because oom_badness() becomes outside of RCU grace > > period upon leaving find_lock_task_mm() when called from proc_oom_score(). > > Hi Tetsuo, > > Yes it is not easy to reproduce seen twice till now and i agree with > your analysis. But David has already fixing this in different way, > So that also looks better to me: > > https://patchwork.kernel.org/patch/10265641/ > Yes, I'm aware of that patch. > But if need to keep that code, So we have to bump up the task > reference that's only i can think of now. I don't think so, for I think it is safe to call has_capability_noaudit(p) with p->alloc_lock held. diff --git a/mm/oom_kill.c b/mm/oom_kill.c index f2e7dfb..4efcfb8 100644 --- a/mm/oom_kill.c +++ b/mm/oom_kill.c @@ -222,7 +222,6 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg, */ points = get_mm_rss(p->mm) + get_mm_counter(p->mm, MM_SWAPENTS) + mm_pgtables_bytes(p->mm) / PAGE_SIZE; - task_unlock(p); /* * Root processes get 3% bonus, just like the __vm_enough_memory() @@ -230,6 +229,7 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg, */ if (has_capability_noaudit(p, CAP_SYS_ADMIN)) points -= (points * 3) / 100; + task_unlock(p); /* Normalize to oom_score_adj units */ adj *= totalpages / 1000;