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

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Date:   Fri, 22 Dec 2017 17:40:10 +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 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.

-- 

Thanks,
Sasha