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Date: Thu, 9 Feb 2017 09:38:25 +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>,
Mateusz Guzik <mguzik@...hat.com>, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v2 1/2] fork: free vmapped stacks in cache when cpus are
offline
On Thu 09-02-17 13:03:46, 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 cached stacks may remain unfreed even when cpu are offline.
> By adding a cpu hotplug callback to free the cached stacks when a cpu
> goes offline, the pages of the cached stacks are not wasted.
>
> Signed-off-by: Hoeun Ryu <hoeun.ryu@...il.com>
> ---
> Changes in v2:
> remove cpuhp callback for `starup`, only `teardown` callback is installed.
>
> kernel/fork.c | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 61284d8..7911ed2 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -170,6 +170,22 @@ void __weak arch_release_thread_stack(unsigned long *stack)
> static DEFINE_PER_CPU(struct vm_struct *, cached_stacks[NR_CACHED_STACKS]);
> #endif
>
> +static int free_vm_stack_cache(unsigned int cpu)
> +{
> + int i;
> +
> + for (i = 0; i < NR_CACHED_STACKS; i++) {
> + struct vm_struct *vm_stack = this_cpu_read(cached_stacks[i]);
the callbak will run on the given cpu so this_cpu_read will be in fact
per_cpu_ptr(cached_stacks[i], cpu). Using this_cpu_read is just too
confusing. Also you do want to make this function defined only for
CONFIG_VMAP_STACK.
> + if (!vm_stack)
> + continue;
> +
> + vfree(vm_stack->addr);
> + this_cpu_write(cached_stacks[i], NULL);
> + }
> +
> + return 0;
> +}
> +
--
Michal Hocko
SUSE Labs
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