linux-kernel - Re: [PATCH v3] fork: free vmapped stacks in cache when cpus are offline

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Date:   Fri, 10 Feb 2017 13:05:24 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Hoeun Ryu <hoeun.ryu@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v3] fork: free vmapped stacks in cache when cpus are
 offline

On Fri 10-02-17 17:32:07, Hoeun Ryu wrote:
[...]
> +static int free_vm_stack_cache(unsigned int cpu)
> +{
> +	struct vm_struct **cached_vm_stacks = per_cpu_ptr(cached_stacks, cpu);
> +	int i;
> +
> +	for (i = 0; i < NR_CACHED_STACKS; i++) {
> +		struct vm_struct **vm_stack = &cached_vm_stacks[i];
> +
> +		if (*vm_stack == NULL)
> +			continue;
> +
> +		vfree((*vm_stack)->addr);
> +		*vm_stack = NULL;

this seems more obscure than necessary. Probably a matter of taste but I
would find the following easier to read
		struct vm_struct *vm_stack = cached_vm_stacks[i];

		if (!vm_stack)
			continue;

		vfree(vm_stack);
		cached_vm_stacks[i] = NULL;

> +	}
> +
> +	return 0;
> +}
>  #endif
>  
>  static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
> @@ -456,6 +474,11 @@ void __init fork_init(void)
>  	for (i = 0; i < UCOUNT_COUNTS; i++) {
>  		init_user_ns.ucount_max[i] = max_threads/2;
>  	}
> +
> +#ifdef CONFIG_VMAP_STACK
> +	cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "vm_stack_cache",
> +			  NULL, free_vm_stack_cache);
> +#endif

I am not familiar the new hotplug infrastructure so I might be missing
something. CPUHP_AP_ONLINE_DYN will allocate a state which is has only
30 slots available. The name also suggests this will be called on an
online event. Why doesn't this have its own state like other users. The
name should also reflect offline event CPUHP_STACK_CACHE_DEAD or
something like that.

-- 
Michal Hocko
SUSE Labs