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

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Message-ID: <20180507143358.GA3071@redhat.com>
Date:   Mon, 7 May 2018 16:33:58 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>, akpm@...ux-foundation.org,
        peterz@...radead.org, viro@...iv.linux.org.uk, mingo@...nel.org,
        paulmck@...ux.vnet.ibm.com, keescook@...omium.org, riel@...hat.com,
        tglx@...utronix.de, kirill.shutemov@...ux.intel.com,
        marcos.souza.org@...il.com, hoeun.ryu@...il.com,
        pasha.tatashin@...cle.com, gs051095@...il.com, dhowells@...hat.com,
        rppt@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
        Balbir Singh <balbir@...ux.vnet.ibm.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] memcg: Replace mm->owner with mm->memcg

On 05/04, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > On 05/04, Eric W. Biederman wrote:
> >>
> >> Oleg Nesterov <oleg@...hat.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.