From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-it0-f70.google.com (mail-it0-f70.google.com [209.85.214.70]) by kanga.kvack.org (Postfix) with ESMTP id DD0556B026F for ; Sun, 15 Jul 2018 04:02:08 -0400 (EDT) Received: by mail-it0-f70.google.com with SMTP id l8-v6so11119316ita.4 for ; Sun, 15 Jul 2018 01:02:08 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id c8-v6sor9821327iog.111.2018.07.15.01.02.07 for (Google Transport Security); Sun, 15 Jul 2018 01:02:07 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: References: <1531557122-12540-1-git-send-email-laoar.shao@gmail.com> From: Yafang Shao Date: Sun, 15 Jul 2018 16:01:26 +0800 Message-ID: Subject: Re: [PATCH] mm: avoid bothering interrupted task when charge memcg in softirq Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Shakeel Butt Cc: Johannes Weiner , Michal Hocko , Vladimir Davydov , Cgroups , Linux MM , LKML On Sun, Jul 15, 2018 at 2:34 PM, Shakeel Butt wrote: > On Sat, Jul 14, 2018 at 10:26 PM Yafang Shao wrote: >> >> On Sun, Jul 15, 2018 at 12:25 PM, Shakeel Butt wrote: >> > On Sat, Jul 14, 2018 at 7:10 PM Yafang Shao wrote: >> >> >> >> On Sat, Jul 14, 2018 at 11:38 PM, Shakeel Butt wrote: >> >> > On Sat, Jul 14, 2018 at 1:32 AM Yafang Shao wrote: >> >> >> >> >> >> try_charge maybe executed in packet receive path, which is in interrupt >> >> >> context. >> >> >> In this situation, the 'current' is the interrupted task, which may has >> >> >> no relation to the rx softirq, So it is nonsense to use 'current'. >> >> >> >> >> > >> >> > Have you actually seen this occurring? >> >> >> >> Hi Shakeel, >> >> >> >> I'm trying to produce this issue, but haven't find it occur yet. >> >> >> >> > I am not very familiar with the >> >> > network code but I can think of two ways try_charge() can be called >> >> > from network code. Either through kmem charging or through >> >> > mem_cgroup_charge_skmem() and both locations correctly handle >> >> > interrupt context. >> >> > >> >> >> >> Why do you say that mem_cgroup_charge_skmem() correctly hanle >> >> interrupt context ? >> >> >> >> Let me show you why mem_cgroup_charge_skmem isn't hanling interrupt >> >> context correctly. >> >> >> >> mem_cgroup_charge_skmem() is calling try_charge() twice. >> >> The first one is with GFP_NOWAIT as the gfp_mask, and the second one >> >> is with (GFP_NOWAIT | __GFP_NOFAIL) as the gfp_mask. >> >> >> >> If page_counter_try_charge() failes at the first time, -ENOMEM is returned. >> >> Then mem_cgroup_charge_skmem() will call try_charge() once more with >> >> __GFP_NOFAIL set, and this time if If page_counter_try_charge() failes >> >> again the ' >> >> force' label in try_charge() will be executed and 0 is returned. >> >> >> >> No matter what, the 'current' will be used and touched, that is >> >> meaning mem_cgroup_charge_skmem() isn't hanling the interrupt context >> >> correctly. >> >> >> > >> > Hi Yafang, >> > >> > If you check mem_cgroup_charge_skmem(), the memcg passed is not >> > 'current' but is from the sock object i.e. sk->sk_memcg for which the >> > network buffer is allocated for. >> > >> >> That's correct, the memcg if from the sock object. >> But the point is, in this situation why 'current' is used in try_charge() ? >> As 'current' is not related with the memcg, which is just a interrupted task. >> > > Hmm so you mean the behavior of memcg charging in the interrupt > context depends on the state of the interrupted task. Yes. > As you have > noted, mem_cgroup_charge_skmem() tries charging again with > __GFP_NOFAIL and the charge succeeds. Basically the memcg charging by > mem_cgroup_charge_skmem() will always succeed irrespective of the > state of the interrupted task. However mem_cgroup_charge_skmem() can > return true if the interrupted task was exiting or a fatal signal is > pending or oom victim or reclaiming memory. Can you please explain why > this is bad? > Let me show you the possible issues cause by this behavoir. 1. In mem_cgroup_oom(), some members in 'current' is set. That means an innocent task will be in task_in_memcg_oom state. But this task may be in a different memcg, I mean the memcg of the 'current' may be differenct with the sk->sk_memcg. Then when this innocent 'current' do try_charge it will hit "if (unlikely(task_in_memcg_oom(current)))" and -ENOMEM is returned, While there're maybe some free memory (or some memory could be freed ) in the memcg of the innocent 'task'. 2. If the interrupted task was exiting or a fatal signal is pending or oom victim, it will directly goto force and 0 is returned, and then mem_cgroup_charge_skmem() will return true. But mem_cgroup_charge_skmem() maybe need to try the second time and return false. That are all unexpected behavoir. At least we must judge that whether the memcg of 'current' is same with sk->sk_memcg if we still want to use current here. Thanks Yafang