linux-kernel - Re: [PATCH 4.14 108/159] kvm, mm: account kvm related kmem slabs to kmemcg

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Message-ID: <20171222180720.mpc7ffshljrce4hf@sasha-lappy>
Date:   Fri, 22 Dec 2017 18:07:23 +0000
From:   alexander.levin@...izon.com
To:     Michal Hocko <mhocko@...nel.org>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 4.14 108/159] kvm, mm: account kvm related kmem slabs to
 kmemcg

On Fri, Dec 22, 2017 at 06:56:16PM +0100, Michal Hocko wrote:
>On Fri 22-12-17 17:40:10, Sasha Levin wrote:
>> On Fri, Dec 22, 2017 at 02:06:07PM +0100, Michal Hocko wrote:
>> >On Fri 22-12-17 13:41:22, Greg KH wrote:
>> >> On Fri, Dec 22, 2017 at 10:34:07AM +0100, Michal Hocko wrote:
>> >> > On Fri 22-12-17 09:46:33, Greg KH wrote:
>> >> > > 4.14-stable review patch.  If anyone has any objections, please let me know.
>> >> > >
>> >> > > ------------------
>> >> > >
>> >> > > From: Shakeel Butt <shakeelb@...gle.com>
>> >> > >
>> >> > >
>> >> > > [ Upstream commit 46bea48ac241fe0b413805952dda74dd0c09ba8b ]
>> >> > >
>> >> > > The kvm slabs can consume a significant amount of system memory
>> >> > > and indeed in our production environment we have observed that
>> >> > > a lot of machines are spending significant amount of memory that
>> >> > > can not be left as system memory overhead. Also the allocations
>> >> > > from these slabs can be triggered directly by user space applications
>> >> > > which has access to kvm and thus a buggy application can leak
>> >> > > such memory. So, these caches should be accounted to kmemcg.
>> >> > >
>> >> > > Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
>> >> > > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> >> > > Signed-off-by: Sasha Levin <alexander.levin@...izon.com>
>> >> > > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> >> >
>> >> > The patch is not marked for stable, neither it fixes an existing bug.
>> >> > It is a nice to have thing for sure but I am wondering how this got
>> >> > through stable-filter.
>> >>
>> >> Sasha picked it out, and it seemed like a sane thing to backport.  If
>> >> you think it's not worthy, I'll gladly drop it, but it seemed like such
>> >> a simple bugfix to include.
>> >
>> >It is not that I would have some specific concerns about this particular
>> >patch. It is more of a worry about the overal process. I thought that
>> >_any_ patch backported to the stable tree would require a specific bug
>> >to be fixed or in exceptional cases a performance issue. I have
>> >experienced this pushback myself when trying to push "no real bug report
>> >but better to have this plugged" patches.
>> >
>> >So something has apparently changed in the process, I just haven't
>> >noticed it. I am worried this might lead to more regression in future.
>> >Not that my worry counts all that much as I am not a stable kernel user
>> >though. So this is just my 2c worth of worry.
>>
>> The way I see it is that stable commits are supposed to fix a bug that
>> a user can hit/exploit, it doesn't have to have an actual user
>> complaining about it.
>>
>> For this particular commit, the way I read it is that a user can avoid
>> his kmemcg limits (maybe maliciously), which would qualify as an
>> actual bug we want to get fixed.
>
>How are you going to judge all the possible relations to other
>subsystems? I mean there is a good reason maintainers mark patches for
>stable trees. How do you want to competently decide this for them? Can
>you do that for all subsystems?
>
>I do not want to underestimate your judgment or misinterpret your
>process here but I _believe_ that picking patches based on the changelog
>without a deep understanding of the subsystem is really risky. We do
>not really have to go a long way to see that. Just look at other patch
>in this very thread [1]. But maybe our our understanding of the stable
>trees are different.

I don't try and override maintainers, I mostly try to get fixes out
of subsystems where maintainers/authors partially (or just don't)
mark their commits for stable.

These patches also go through a much longer review process than
commits that are marked for stable (there are at least 3 emails issued
for each such commit, and at least 1 week (usually much more) is
given for reviews).

-- 

Thanks,
Sasha