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Subject: Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints
To:     Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Luiz Capitulino <lcapitulino@redhat.com>,
        Alexander Duyck <alexander.duyck@gmail.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, rkrcmar@redhat.com, x86@kernel.org,
        mingo@redhat.com, bp@alien8.de, hpa@zytor.com, pbonzini@redhat.com,
        tglx@linutronix.de, akpm@linux-foundation.org
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From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Message-ID: <5f5c03ac-0d21-d92b-1772-f26773437019@redhat.com>
Date:   Fri, 8 Feb 2019 16:05:48 -0500
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From: Nitesh Narayan Lal <nitesh@redhat.com>
To: Alexander Duyck <alexander.h.duyck@linux.intel.com>,
 Luiz Capitulino <lcapitulino@redhat.com>,
 Alexander Duyck <alexander.duyck@gmail.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
 rkrcmar@redhat.com, x86@kernel.org, mingo@redhat.com, bp@alien8.de,
 hpa@zytor.com, pbonzini@redhat.com, tglx@linutronix.de,
 akpm@linux-foundation.org
Message-ID: <5f5c03ac-0d21-d92b-1772-f26773437019@redhat.com>
Subject: Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints
References: <20190204181118.12095.38300.stgit@localhost.localdomain>
 <20190204181552.12095.46287.stgit@localhost.localdomain>
 <20190207132104.17a296da@doriath>
 <34c93e5a05a7dc93e38364733f8832f2e1b2dcb3.camel@linux.intel.com>
In-Reply-To: <34c93e5a05a7dc93e38364733f8832f2e1b2dcb3.camel@linux.intel.com>

--DbNdtYN4o8nLIgLlGD1ViUxA0kuqQwqic
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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@gmail.com> wrote:
>>
>>> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>>
>>> Add guest support for providing free memory hints to the KVM hypervis=
or 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 obvio=
us
>>> 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 do=
ing an
>>> MADV_DONTNEED would effectively negate the hint, so it would be wasti=
ng
>>> cycles to do so.
>>>
>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>> ---
>>>  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 @@
>>> =20
>>>  struct page;
>>> =20
>>> +#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 or=
der)
>>> +{
>>> +	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>
>>> =20
>>>  static int kvmapf =3D 1;
>>> +DEFINE_STATIC_KEY_FALSE(pv_free_page_hint_enabled);
>>> =20
>>>  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);
>>> =20
>>> +	/*
>>> +	 * 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 =3D kvm_smp_prepare_cpus;
>>>  	smp_ops.smp_prepare_boot_cpu =3D 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);
>>> =20
>>> +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?

--=20
Nitesh


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