From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f72.google.com (mail-pl0-f72.google.com [209.85.160.72]) by kanga.kvack.org (Postfix) with ESMTP id 95B826B000A for ; Mon, 6 Aug 2018 10:58:23 -0400 (EDT) Received: by mail-pl0-f72.google.com with SMTP id w18-v6so8693761plp.3 for ; Mon, 06 Aug 2018 07:58:23 -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 c18-v6sor3158495pgd.80.2018.08.06.07.58.22 for (Google Transport Security); Mon, 06 Aug 2018 07:58:22 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20180806142124.GP19540@dhcp22.suse.cz> References: <0000000000005e979605729c1564@google.com> <20180806091552.GE19540@dhcp22.suse.cz> <20180806094827.GH19540@dhcp22.suse.cz> <20180806110224.GI19540@dhcp22.suse.cz> <20180806142124.GP19540@dhcp22.suse.cz> From: Dmitry Vyukov Date: Mon, 6 Aug 2018 16:58:01 +0200 Message-ID: Subject: Re: WARNING in try_charge Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: syzbot , cgroups@vger.kernel.org, Johannes Weiner , LKML , Linux-MM , syzkaller-bugs , Vladimir Davydov , Dmitry Torokhov On Mon, Aug 6, 2018 at 4:21 PM, Michal Hocko wrote: > On Mon 06-08-18 13:57:38, Dmitry Vyukov wrote: >> On Mon, Aug 6, 2018 at 1:02 PM, Michal Hocko wrote: > [...] >> >> A much >> >> friendlier for user way to say this would be print a message at the >> >> point of misconfiguration saying what exactly is wrong, e.g. "pid $PID >> >> misconfigures cgroup /cgroup/path with mem.limit=0" without a stack >> >> trace (does not give any useful info for user). And return EINVAL if >> >> it can't fly at all? And then leave the "or a kernel bug" part for the >> >> WARNING each occurrence of which we do want to be reported to kernel >> >> developers. >> > >> > But this is not applicable here. Your misconfiguration is quite obvious >> > because you simply set the hard limit to 0. This is not the only >> > situation when this can happen. There is no clear point to tell, you are >> > doing this wrong. If it was we would do it at that point obviously. >> >> But, isn't there a point were hard limit is set to 0? I would expect >> there is a something like cgroup file write handler with a value of 0 >> or something. > > Yeah, but this is only one instance of the problem. Other is that the > memcg is not reclaimable for any other reasons. And we do not know what > those might be > >> >> > If you have a strong reason to believe that this is an abuse of WARN I >> > am all happy to change that. But I haven't heard any yet, to be honest. >> >> WARN must not be used for anything that is not kernel bugs. If this is >> not kernel bug, WARN must not be used here. > > This is rather strong wording without any backing arguments. I strongly > doubt 90% of existing WARN* match this expectation. WARN* has > traditionally been a way to tell that something suspicious is going on. > Those situation are mostly likely not fatal but it is good to know they > are happening. > > Sure there is that panic_on_warn thingy which you seem to be using and I > suspect it is a reason why you are so careful about warnings in general > but my experience tells me that this configuration is barely usable > except for testing (which is your case). > > But as I've said, I do not insist on WARN here. All I care about is to > warn user that something might go south and this may be either due to > misconfiguration or a subtly wrong memcg reclaim/OOM handler behavior. I am a bit lost. Can limit=0 legally lead to the warnings? Or there is also a kernel bug on top of that and it's actually a kernel bug that provokes the warning? If it's a kernel bug, then I propose to stop arguing about configuration and concentrate on the bug. If it's just the misconfiguration that triggers the warning, then can we separate the 2 causes of the warning (user misconfiguration and kernel bugs)? Say, return EINVAL when mem limit is set to 0 (and print a line to console if necessary)? Or if the limit=0 is somehow not possible/desirable to detect right away, check limit=0 at the point of the warning and don't want?