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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Kirill Tkhai <ktkhai@virtuozzo.com>, akpm@linux-foundation.org,
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        Balbir Singh <balbir@linux.vnet.ibm.com>,
        Tejun Heo <tj@kernel.org>
References: <20180503133338.GA23401@redhat.com> <87y3h0x0qg.fsf@xmission.com>
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        <20180507143358.GA3071@redhat.com> <87vabyvnw0.fsf@xmission.com>
        <20180509144016.GA25742@redhat.com>
Date:   Wed, 09 May 2018 22:09:25 -0500
In-Reply-To: <20180509144016.GA25742@redhat.com> (Oleg Nesterov's message of
        "Wed, 9 May 2018 16:40:16 +0200")
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Subject: Re: [PATCH] memcg: Replace mm->owner with mm->memcg
Sender: linux-kernel-owner@vger.kernel.org
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Oleg Nesterov <oleg@redhat.com> writes:

> On 05/07, Eric W. Biederman wrote:
>>
>> Oleg Nesterov <oleg@redhat.com> writes:
>>
>> > before your patch get_mem_cgroup_from_mm() looks at mm->owner == current
>> > (in this case) and mem_cgroup_from_task() should return the correct memcg
>> > even if execing task migrates after bprm_mm_init(). At least in the common
>> > case when the old mm is not shared.
>> >
>> > After your patch the memory allocations in copy_strings() won't be accounted
>> > correctly, bprm->mm->memcg is wrong if this task migrates. And iiuc your recent
>> > "[PATCH 2/2] memcg: Close the race between migration and installing bprm->mm as mm"
>> > doesn't fix the problem.
>> >
>> > No?
>>
>> The patch does solve the issue.  There should be nothing a userspace
>> process can observe that should tell it where in the middle of exec
>> such a migration happend so placing the migration at what from the
>> kernel's perspective might be technically later should not be a problem.
>>
>> If it is a problem the issue is that there is a way to observe the
>> difference.
>
> So. The task migrates from some MEMCG right after bprm_mm_init().
>
> copy_strings() triggers OOM in MEMCG. This is quite possible, it can use a lot
> of memory and that is why we have acct_arg_size() to make these allocations
> visible to oom killer.
>
> task_in_mem_cgroup(MEMCG) returns false and oom killer has to kill another
> innocent process in MEMCG.
>
> Does this look like a way to observe the difference?

Sort of.

I don't know how the memcg gets away without migrating charges
when it migrates a process.  With charges not being migrated
I don't think this is observable.

That does look like a real issue however.

>> > Perhaps we can change get_mem_cgroup_from_mm() to use
>> > mem_cgroup_from_css(current, memory_cgrp_id) if mm->memcg == NULL?
>>
>> Please God no.  Having any unnecessary special case is just going to
>> confuse people and cause bugs.
>
> To me the unnecessary special case is the new_mm->memcg which is used for
> accounting but doesn't follow migration till exec_mmap(). But I won't
> argue.

Eric