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

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Date:   Fri, 04 May 2018 10:49:42 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.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

Oleg Nesterov <oleg@...hat.com> writes:

> On 05/04, Eric W. Biederman wrote:
>>
>> Oleg Nesterov <oleg@...hat.com> writes:
>>
>> > OK, what about exec() ? mm_init_memcg() initializes bprm->mm->memcg early in
>> > bprm_mm_init(). What if the execing task migrates before exec_mmap() ?
>>
>> We need the the cgroup when the mm is initialized.  That way we have the
>> cgroup information when initializing the mm.
>
> Yes, we need to initialize new_mm->memcg but iiuc mostly for the error path,
>
>> I don't know if a lock preventing changing the cgroup in exec or just
>> a little bit of code in exec_mmap to ensure mm->memcg is properly set
>> is the better approach.
>
> 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
                      
I am surprised I can't easily find more.   Apparently in load_elf_binary
we call elf_mmap after set_new_exec and install_exec_creds, making
a gracefull recovery from elf_mmap failures impossible.

In any case we most definitely need the memory control group properly
setup before exec_mmap.

Eric