linux-kernel - Re: [PATCH v2 1/2] fork: free vmapped stacks in cache when cpus are offline

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Date:   Thu, 9 Feb 2017 22:36:04 +0900
From:   Hoeun Ryu <hoeun.ryu@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
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, Feb 9, 2017 at 5:38 PM, Michal Hocko <mhocko@...nel.org> wrote:
> 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.
>

Sorry, Thank you for your correction. I will fix this.

>> +             if (!vm_stack)
>> +                     continue;
>> +
>> +             vfree(vm_stack->addr);
>> +             this_cpu_write(cached_stacks[i], NULL);
>> +     }
>> +
>> +     return 0;
>> +}
>> +
>
> --
> Michal Hocko
> SUSE Labs