Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints

[Nitesh Narayan Lal]

On 2/7/19 1:44 PM, Alexander Duyck wrote:
> On Thu, 2019-02-07 at 13:21 -0500, Luiz Capitulino wrote:
>> On Mon, 04 Feb 2019 10:15:52 -0800
>> Alexander Duyck <alexander.duyck@xxxxxxxxx> wrote:
>>
>>> From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>>>
>>> Add guest support for providing free memory hints to the KVM hypervisor for
>>> freed pages huge TLB size or larger. I am restricting the size to
>>> huge TLB order and larger because the hypercalls are too expensive to be
>>> performing one per 4K page. Using the huge TLB order became the obvious
>>> choice for the order to use as it allows us to avoid fragmentation of higher
>>> order memory on the host.
>>>
>>> I have limited the functionality so that it doesn't work when page
>>> poisoning is enabled. I did this because a write to the page after doing an
>>> MADV_DONTNEED would effectively negate the hint, so it would be wasting
>>> cycles to do so.
>>>
>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>>> ---
>>>  arch/x86/include/asm/page.h |   13 +++++++++++++
>>>  arch/x86/kernel/kvm.c       |   23 +++++++++++++++++++++++
>>>  2 files changed, 36 insertions(+)
>>>
>>> diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
>>> index 7555b48803a8..4487ad7a3385 100644
>>> --- a/arch/x86/include/asm/page.h
>>> +++ b/arch/x86/include/asm/page.h
>>> @@ -18,6 +18,19 @@
>>>  
>>>  struct page;
>>>  
>>> +#ifdef CONFIG_KVM_GUEST
>>> +#include <linux/jump_label.h>
>>> +extern struct static_key_false pv_free_page_hint_enabled;
>>> +
>>> +#define HAVE_ARCH_FREE_PAGE
>>> +void __arch_free_page(struct page *page, unsigned int order);
>>> +static inline void arch_free_page(struct page *page, unsigned int order)
>>> +{
>>> +	if (static_branch_unlikely(&pv_free_page_hint_enabled))
>>> +		__arch_free_page(page, order);
>>> +}
>>> +#endif
>>> +
>>>  #include <linux/range.h>
>>>  extern struct range pfn_mapped[];
>>>  extern int nr_pfn_mapped;
>>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>>> index 5c93a65ee1e5..09c91641c36c 100644
>>> --- a/arch/x86/kernel/kvm.c
>>> +++ b/arch/x86/kernel/kvm.c
>>> @@ -48,6 +48,7 @@
>>>  #include <asm/tlb.h>
>>>  
>>>  static int kvmapf = 1;
>>> +DEFINE_STATIC_KEY_FALSE(pv_free_page_hint_enabled);
>>>  
>>>  static int __init parse_no_kvmapf(char *arg)
>>>  {
>>> @@ -648,6 +649,15 @@ static void __init kvm_guest_init(void)
>>>  	if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
>>>  		apic_set_eoi_write(kvm_guest_apic_eoi_write);
>>>  
>>> +	/*
>>> +	 * The free page hinting doesn't add much value if page poisoning
>>> +	 * is enabled. So we only enable the feature if page poisoning is
>>> +	 * no present.
>>> +	 */
>>> +	if (!page_poisoning_enabled() &&
>>> +	    kvm_para_has_feature(KVM_FEATURE_PV_UNUSED_PAGE_HINT))
>>> +		static_branch_enable(&pv_free_page_hint_enabled);
>>> +
>>>  #ifdef CONFIG_SMP
>>>  	smp_ops.smp_prepare_cpus = kvm_smp_prepare_cpus;
>>>  	smp_ops.smp_prepare_boot_cpu = kvm_smp_prepare_boot_cpu;
>>> @@ -762,6 +772,19 @@ static __init int kvm_setup_pv_tlb_flush(void)
>>>  }
>>>  arch_initcall(kvm_setup_pv_tlb_flush);
>>>  
>>> +void __arch_free_page(struct page *page, unsigned int order)
>>> +{
>>> +	/*
>>> +	 * Limit hints to blocks no smaller than pageblock in
>>> +	 * size to limit the cost for the hypercalls.
>>> +	 */
>>> +	if (order < KVM_PV_UNUSED_PAGE_HINT_MIN_ORDER)
>>> +		return;
>>> +
>>> +	kvm_hypercall2(KVM_HC_UNUSED_PAGE_HINT, page_to_phys(page),
>>> +		       PAGE_SIZE << order);
>> Does this mean that the vCPU executing this will get stuck
>> here for the duration of the hypercall? Isn't that too long,
>> considering that the zone lock is taken and madvise in the
>> host block on semaphores?
> I'm pretty sure the zone lock isn't held when this is called. The lock
> isn't acquired until later in the path. This gets executed just before
> the page poisoning call which would take time as well since it would
> have to memset an entire page. This function is called as a part of
> free_pages_prepare, the zone locks aren't acquired until we are calling
> into either free_one_page and a few spots before calling
> __free_one_page.
>
> My other function in patch 4 which does this from inside of
> __free_one_page does have to release the zone lock since it is taken
> there.
>
Considering hypercall's are costly, will it not make sense to coalesce
the pages you are reporting and make a single hypercall for a bunch of
pages?

-- 
Nitesh

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