Re: [RFC][Patch v8 2/7] KVM: Enabling guest free page hinting via static key

[Nitesh Narayan Lal]

On 2/8/19 1:07 PM, Alexander Duyck wrote:
> On Mon, Feb 4, 2019 at 2:11 PM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote:
>> This patch enables the guest free page hinting support
>> to enable or disable based on the STATIC key which
>> could be set via sysctl.
>>
>> Signed-off-by: Nitesh Narayan Lal <nitesh@xxxxxxxxxx>
>> ---
>>  include/linux/gfp.h          |  2 ++
>>  include/linux/page_hinting.h |  5 +++++
>>  kernel/sysctl.c              |  9 +++++++++
>>  virt/kvm/page_hinting.c      | 23 +++++++++++++++++++++++
>>  4 files changed, 39 insertions(+)
>>
>> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
>> index e596527284ba..8389219a076a 100644
>> --- a/include/linux/gfp.h
>> +++ b/include/linux/gfp.h
>> @@ -461,6 +461,8 @@ static inline struct zonelist *node_zonelist(int nid, gfp_t flags)
>>  #define HAVE_ARCH_FREE_PAGE
>>  static inline void arch_free_page(struct page *page, int order)
>>  {
>> +       if (!static_branch_unlikely(&guest_page_hinting_key))
>> +               return;
>>         guest_free_page(page, order);
>>  }
>>  #endif
>> diff --git a/include/linux/page_hinting.h b/include/linux/page_hinting.h
>> index b54f7428f348..9bdcf63e1306 100644
>> --- a/include/linux/page_hinting.h
>> +++ b/include/linux/page_hinting.h
>> @@ -14,4 +14,9 @@ struct hypervisor_pages {
>>         unsigned int order;
>>  };
>>
>> +extern int guest_page_hinting_flag;
>> +extern struct static_key_false guest_page_hinting_key;
>> +
>> +int guest_page_hinting_sysctl(struct ctl_table *table, int write,
>> +                             void __user *buffer, size_t *lenp, loff_t *ppos);
>>  void guest_free_page(struct page *page, int order);
>> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
>> index ba4d9e85feb8..5d53629c9bfb 100644
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -1690,6 +1690,15 @@ static struct ctl_table vm_table[] = {
>>                 .extra1         = (void *)&mmap_rnd_compat_bits_min,
>>                 .extra2         = (void *)&mmap_rnd_compat_bits_max,
>>         },
>> +#endif
>> +#ifdef CONFIG_KVM_FREE_PAGE_HINTING
>> +       {
>> +               .procname       = "guest-page-hinting",
>> +               .data           = &guest_page_hinting_flag,
>> +               .maxlen         = sizeof(guest_page_hinting_flag),
>> +               .mode           = 0644,
>> +               .proc_handler   = guest_page_hinting_sysctl,
>> +       },
>>  #endif
>>         { }
>>  };
> Since you are adding a new sysctl shouldn't you also be updating
> Documentation/sysctl/vm.txt?
Indeed I will be doing that.
However, I would first like to close other major issues with the design.
>
>> diff --git a/virt/kvm/page_hinting.c b/virt/kvm/page_hinting.c
>> index 818bd6b84e0c..4a34ea8db0c8 100644
>> --- a/virt/kvm/page_hinting.c
>> +++ b/virt/kvm/page_hinting.c
>> @@ -1,6 +1,7 @@
>>  #include <linux/gfp.h>
>>  #include <linux/mm.h>
>>  #include <linux/kernel.h>
>> +#include <linux/kvm_host.h>
>>
>>  /*
>>   * struct kvm_free_pages - Tracks the pages which are freed by the guest.
>> @@ -31,6 +32,28 @@ struct page_hinting {
>>
>>  DEFINE_PER_CPU(struct page_hinting, hinting_obj);
>>
>> +struct static_key_false guest_page_hinting_key  = STATIC_KEY_FALSE_INIT;
>> +EXPORT_SYMBOL(guest_page_hinting_key);
>> +static DEFINE_MUTEX(hinting_mutex);
>> +int guest_page_hinting_flag;
>> +EXPORT_SYMBOL(guest_page_hinting_flag);
> I'm not entirely sure this flag makes sense to me. What is to prevent
> someone from turning this on when there is no means of actually using
> the hints. I understand right now that guest_free_page doesn't
> actually do anything, but when it does I would assume it has to
> interact with a device. If that device is not present would it still
> make sense for us to be generating hints?
Fair point, I will address this issue.
>
>> +
>> +int guest_page_hinting_sysctl(struct ctl_table *table, int write,
>> +                             void __user *buffer, size_t *lenp,
>> +                             loff_t *ppos)
>> +{
>> +       int ret;
>> +
>> +       mutex_lock(&hinting_mutex);
>> +       ret = proc_dointvec(table, write, buffer, lenp, ppos);
>> +       if (guest_page_hinting_flag)
>> +               static_key_enable(&guest_page_hinting_key.key);
>> +       else
>> +               static_key_disable(&guest_page_hinting_key.key);
>> +       mutex_unlock(&hinting_mutex);
>> +       return ret;
>> +}
>> +
>>  void guest_free_page(struct page *page, int order)
>>  {
>>  }
>> --
>> 2.17.2
>>
-- 
Regards
Nitesh

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