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SubjectRe: [PATCH] memcg: Replace mm->owner with mm->memcg
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

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