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Date: Mon, 7 May 2018 16:33:58 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@kernel.org>,
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Subject: Re: [PATCH] memcg: Replace mm->owner with mm->memcg
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On 05/04, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@redhat.com> writes:
>
> > On 05/04, Eric W. Biederman wrote:
> >>
> >> Oleg Nesterov <oleg@redhat.com> writes:
> >>
> >> > I'd vote for the change in exec_mmap(). This way mm_init_memcg() can just
> >> > nullify mm->memcg.
> >>
> >> There is at least one common path where we need the memory control group
> >> properly initialized so memory allocations don't escape the memory
> >> control group.
> >>
> >> do_execveat_common
> >>    copy_strings
> >>       get_arg_page
> >>          get_user_pages_remote
> >>             __get_user_pages_locked
> >>                __get_user_pages
> >>                   faultin_page
> >>                      handle_mm_fault
> >>                         count_memcg_event_mm
> >>                         __handle_mm_fault
> >>                           handle_pte_fault
> >>                              do_anonymous_page
> >>                                 mem_cgroup_try_charge

Ah yes, indeed.

> mm_init_memcg is at the same point as mm_init_owner.  So my change did
> not introduce any logic changes on when the memory control group became
> valid.

Not sure, but perhaps I am all confused....

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?

Perhaps we can change get_mem_cgroup_from_mm() to use
mem_cgroup_from_css(current, memory_cgrp_id) if mm->memcg == NULL?

Oleg.