linux-kernel - Re: Mount structures are leaked

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Date:   Thu, 8 Jun 2017 15:48:10 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Kirill Tkhai <ktkhai@...tuozzo.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: Mount structures are leaked

On Thu, Jun 8, 2017 at 2:37 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Thu, Jun 08, 2017 at 01:49:38PM -0700, Andrei Vagin wrote:
>> Hello,
>>
>> We found that mount structures are leaked on the upstream linux kernel:
>>
>> [root@...m criu]# cat /proc/slabinfo | grep mnt
>> mnt_cache          36456  36456    384   42    4 : tunables    0    0
>>   0 : slabdata    868    868      0
>> [root@...m criu]#  python test/zdtm.py run  -t zdtm/static/env00
>> --iter 10 -f ns
>> === Run 1/1 ================ zdtm/static/env00
>>
>> ========================= Run zdtm/static/env00 in ns ==========================
>> Start test
>> ./env00 --pidfile=env00.pid --outfile=env00.out --envname=ENV_00_TEST
>> Run criu dump
>> Run criu restore
>> Run criu dump
>> ....
>> Run criu restore
>> Send the 15 signal to  339
>> Wait for zdtm/static/env00(339) to die for 0.100000
>> Removing dump/zdtm/static/env00/31
>> ========================= Test zdtm/static/env00 PASS ==========================
>> [root@...m criu]# cat /proc/slabinfo | grep mnt
>> mnt_cache          36834  36834    384   42    4 : tunables    0    0
>>   0 : slabdata    877    877      0
>>
>> [root@...m linux]# git describe HEAD
>> v4.12-rc4-122-gb29794e
>>
>> [root@...m ~]# uname -a
>> Linux zdtm.openvz.org 4.12.0-rc4+ #2 SMP Thu Jun 8 20:49:01 CEST 2017
>> x86_64 x86_64 x86_64 GNU/Linux
>
> For fsck sake...  Andrei, you *do* know better.
>         1) I have no idea what setup do you have - e.g. whether you have mount event
> propagation set up in a way that ends up with mounts accumulating somewhere.
>         2) I have no idea what those scripts are and names don't look descriptive
> enough to google for them in hope to find out (nor the version of those scripts,
> if there had been more than one)
>         3) I have no idea which config do you have.
>         4) I have no idea which kernel is that about, other than "rc4 with something
> on top of it"
>         5) I have no idea how that had behaved on other kernels (or how that was
> supposed to behave in the first place)
>
> So it boils down to "we've done something, it has given a result we didn't expect,
> the kernel must've been broken".  About the only thing I can suggest at that point is
> telnet bofh.jeffballard.us 666
> and see if it provides an inspiration...

Hi All,

I'm sorry for this stripped report. I continue investigating this
issue and soon I will provide more info about it.

I found that there is one more suspicious slab:

[root@...m criu]# cat /proc/slabinfo | grep ^kmalloc-32
kmalloc-32         49024  49152     32  128    1 : tunables    0    0
  0 : slabdata    384    384      0

I tried to use the kmemleak detector, but it reports nothing useful in
this case.

 test/zdtm.py is a script, which we use to execute criu tests:
https://github.com/xemul/criu

$ python test/zdtm.py run  -t zdtm/static/env00 -f ns --iter 10

This command executes a test in a new set on namespaces (net, mnt,
pid, ipc, uts), then it dumps and restores this test container ten
times and check that everything works as expected. The env00 test sets
an environment variable and then check that the variable has the same
value after c/r.