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

[Alexander Duyck <alexander.duyck@gmail.com>]

On Mon, Feb 4, 2019 at 2:11 PM Nitesh Narayan Lal <nitesh@redhat.com> 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@redhat.com>
> ---
>  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?

> 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?

> +
> +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
>