kernel-hardening - Re: [PATCH 1/3] fork: dynamically allocate cache array for vmapped stacks using cpuhp

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Message-ID: <CALCETrXruGdh3Z_kTnp8fUBr6J3a4bosumQu38ygM37SSD5EfQ@mail.gmail.com>
Date: Fri, 3 Feb 2017 09:52:57 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Hoeun Ryu <hoeun.ryu@...il.com>
Cc: Michal Hocko <mhocko@...nel.org>, 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>, Mateusz Guzik <mguzik@...hat.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 1/3] fork: dynamically allocate cache array for vmapped
 stacks using cpuhp

On Fri, Feb 3, 2017 at 8:42 AM, Hoeun Ryu <hoeun.ryu@...il.com> wrote:
> On Sat, Feb 4, 2017 at 12:39 AM, Michal Hocko <mhocko@...nel.org> wrote:
>> On Sat 04-02-17 00:30:05, Hoeun Ryu wrote:
>>>  Using virtually mapped stack, kernel stacks are allocated via vmalloc.
>>> In the current implementation, two stacks per cpu can be cached when
>>> tasks are freed and the cached stacks are used again in task duplications.
>>> but the array for the cached stacks is statically allocated by per-cpu api.
>>>  In this new implementation, the array for the cached stacks are dynamically
>>> allocted and freed by cpu hotplug callbacks and the cached stacks are freed
>>> when cpu is down. setup for cpu hotplug is established in fork_init().
>>
>> Why do we want this? I can see that the follow up patch makes the number
>> configurable but the changelog doesn't describe the motivation for that.
>> Which workload would benefit from a higher value?
>>
>
> The key difference of this implementation, the cached stacks for a cpu
> is freed when a cpu is down.
> so the cached stacks are no longer wasted.
> In the current implementation, the cached stacks for a cpu still
> remain on the system when a cpu is down.
> I think we could imagine what if a machine has many cpus and someone
> wants to have bigger size of stack caches.

Then how about just registering a simple hotplug hook to free the
stacks without worrying about freeing the tiny array as well?

--Andy

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