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

[Oleg Nesterov]

On 05/07, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@xxxxxxxxxx> 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?

> > 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.

Oleg.