Re: [RFC PATCH 1/2] mm: rework memcg kernel stack accounting

[Andy Lutomirski <luto@amacapital.net>]

> On Aug 15, 2018, at 9:55 AM, Roman Gushchin <guro@fb.com> wrote:
> 
>> On Wed, Aug 15, 2018 at 12:39:23PM -0400, Johannes Weiner wrote:
>>> On Tue, Aug 14, 2018 at 05:36:19PM -0700, Roman Gushchin wrote:
>>> @@ -224,9 +224,14 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
>>>        return s->addr;
>>>    }
>>> 
>>> +    /*
>>> +     * Allocated stacks are cached and later reused by new threads,
>>> +     * so memcg accounting is performed manually on assigning/releasing
>>> +     * stacks to tasks. Drop __GFP_ACCOUNT.
>>> +     */
>>>    stack = __vmalloc_node_range(THREAD_SIZE, THREAD_ALIGN,
>>>                     VMALLOC_START, VMALLOC_END,
>>> -                     THREADINFO_GFP,
>>> +                     THREADINFO_GFP & ~__GFP_ACCOUNT,
>>>                     PAGE_KERNEL,
>>>                     0, node, __builtin_return_address(0));
>>> 
>>> @@ -246,12 +251,41 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
>>> #endif
>>> }
>>> 
>>> +static void memcg_charge_kernel_stack(struct task_struct *tsk)
>>> +{
>>> +#ifdef CONFIG_VMAP_STACK
>>> +    struct vm_struct *vm = task_stack_vm_area(tsk);
>>> +
>>> +    if (vm) {
>>> +        int i;
>>> +
>>> +        for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
>>> +            memcg_kmem_charge(vm->pages[i], __GFP_NOFAIL,
>>> +                      compound_order(vm->pages[i]));
>>> +
>>> +        /* All stack pages belong to the same memcg. */
>>> +        mod_memcg_page_state(vm->pages[0], MEMCG_KERNEL_STACK_KB,
>>> +                     THREAD_SIZE / 1024);
>>> +    }
>>> +#endif
>>> +}
>> 
>> Before this change, the memory limit can fail the fork, but afterwards
>> fork() can grow memory consumption unimpeded by the cgroup settings.
>> 
>> Can we continue to use try_charge() here and fail the fork?
> 
> We can, but I'm not convinced we should.
> 
> Kernel stack is relatively small, and it's already allocated at this point.
> So IMO exceeding the memcg limit for 1-2 pages isn't worse than
> adding complexity and handle this case (e.g. uncharge partially
> charged stack). Do you have an example, when it does matter?

What bounds it to just a few pages?  Couldn’t there be lots of forks in flight that all hit this path?  It’s unlikely, and there are surely easier DoS vectors, but still.