linux-kernel - Re: [mm] 4e2c82a409: ltp.overcommit

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Date:   Tue, 7 Jul 2020 07:43:48 -0400
From:   Qian Cai <cai@....pw>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Feng Tang <feng.tang@...el.com>,
        kernel test robot <rong.a.chen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Kees Cook <keescook@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Iurii Zaikin <yzaikin@...gle.com>, andi.kleen@...el.com,
        tim.c.chen@...el.com, dave.hansen@...el.com, ying.huang@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org, lkp@...ts.01.org
Subject: Re: [mm] 4e2c82a409: ltp.overcommit_memory01.fail

> On Jul 7, 2020, at 6:28 AM, Michal Hocko <mhocko@...nel.org> wrote:
> 
> Would you have any examples? Because I find this highly unlikely.
> OVERCOMMIT_NEVER only works when virtual memory is not largerly
> overcommited wrt to real memory demand. And that tends to be more of
> an exception rather than a rule. "Modern" userspace (whatever that
> means) tends to be really hungry with virtual memory which is only used
> very sparsely.
> 
> I would argue that either somebody is running an "OVERCOMMIT_NEVER"
> friendly SW and this is a permanent setting or this is not used at all.
> At least this is my experience.
> 
> So I strongly suspect that LTP test failure is not something we should
> really lose sleep over. It would be nice to find a way to flush existing
> batches but I would rather see a real workload that would suffer from
> this imprecision.

I hear you many times that you really don’t care about those use cases unless you hear exactly people are using in your world.

For example, when you said LTP oom tests are totally artificial last time and how less you care about if they are failing, and I could only enjoy their efficiencies to find many issues like race conditions and bad error accumulation handling etc that your “real world use cases” are going to take ages or no way to flag them.

There are just too many valid use cases in this wild world. The difference is that I admit that I don’t know or even aware all the use cases, and I don’t believe you do as well.

If a patchset broke the existing behaviors that written exactly in the spec, it is then someone has to prove its innocent. For example, if nobody is going to rely on something like this now and future, and then fix the spec and explain exactly nobody should be rely upon.