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Date:   Wed, 15 Aug 2018 10:25:58 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Andy Lutomirski <luto@amacapital.net>
CC:     Johannes Weiner <hannes@cmpxchg.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>,
        Michal Hocko <mhocko@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Konstantin Khlebnikov <koct9i@gmail.com>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [RFC PATCH 1/2] mm: rework memcg kernel stack accounting
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On Wed, Aug 15, 2018 at 10:12:42AM -0700, Andy Lutomirski wrote:
> 
> 
> > 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.

Because any following memcg-aware allocation will fail.
There is also the pid cgroup controlled which can be used to limit the number
of forks.

Anyway, I'm ok to handle the this case and fail fork,
if you think it does matter.