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

Michal Hocko
2017-12-22T17:56:22Z

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@google.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@google.com>
> >> > > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> >> > > Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
> >> > > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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.

[1] http://lkml.kernel.org/r/20171222141810.dpeozmylmnj253do@xps
--
Michal Hocko
SUSE Labs