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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@redhat.com>
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Date:   Fri, 04 May 2018 10:49:42 -0500
In-Reply-To: <20180504145435.GA26573@redhat.com> (Oleg Nesterov's message of
        "Fri, 4 May 2018 16:54:35 +0200")
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Subject: Re: [PATCH] memcg: Replace mm->owner with mm->memcg
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Oleg Nesterov <oleg@redhat.com> writes:

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