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From:   Nitesh Narayan Lal <nitesh@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Alexander Duyck <alexander.duyck@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        rkrcmar@redhat.com, alexander.h.duyck@linux.intel.com,
        x86@kernel.org, mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
        pbonzini@redhat.com, tglx@linutronix.de, akpm@linux-foundation.org
Subject: Re: [RFC PATCH 4/4] mm: Add merge page notifier
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Date:   Mon, 11 Feb 2019 13:09:40 -0500
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From: Nitesh Narayan Lal <nitesh@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, kvm@vger.kernel.org, rkrcmar@redhat.com,
 alexander.h.duyck@linux.intel.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: <ea3ab38d-36e7-d2fa-6ebf-df6c048e17e0@redhat.com>
Subject: Re: [RFC PATCH 4/4] mm: Add merge page notifier

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On 2/11/19 12:41 PM, Michael S. Tsirkin wrote:
> On Mon, Feb 11, 2019 at 11:24:02AM -0500, Nitesh Narayan Lal wrote:
>> On 2/11/19 9:17 AM, Michael S. Tsirkin wrote:
>>> On Mon, Feb 11, 2019 at 08:30:03AM -0500, Nitesh Narayan Lal wrote:
>>>> On 2/9/19 7:57 PM, Michael S. Tsirkin wrote:
>>>>> On Mon, Feb 04, 2019 at 10:15:58AM -0800, Alexander Duyck wrote:
>>>>>> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>>>>>
>>>>>> Because the implementation was limiting itself to only providing h=
ints on
>>>>>> pages huge TLB order sized or larger we introduced the possibility=
 for free
>>>>>> pages to slip past us because they are freed as something less the=
n
>>>>>> huge TLB in size and aggregated with buddies later.
>>>>>>
>>>>>> To address that I am adding a new call arch_merge_page which is ca=
lled
>>>>>> after __free_one_page has merged a pair of pages to create a highe=
r order
>>>>>> page. By doing this I am able to fill the gap and provide full cov=
erage for
>>>>>> all of the pages huge TLB order or larger.
>>>>>>
>>>>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>=

>>>>> Looks like this will be helpful whenever active free page
>>>>> hints are added. So I think it's a good idea to
>>>>> add a hook.
>>>>>
>>>>> However, could you split adding the hook to a separate
>>>>> patch from the KVM hypercall based implementation?
>>>>>
>>>>> Then e.g. Nilal's patches could reuse it too.
>>>> With the current design of my patch-set, if I use this hook to repor=
t
>>>> free pages. I will end up making redundant hints for the same pfns.
>>>>
>>>> This is because the pages once freed by the host, are returned back =
to
>>>> the buddy.
>>> Suggestions on how you'd like to fix this? You do need this if
>>> you introduce a size cut-off right?
>> I do, there are two ways to go about it.
>>
>> One is to=C2=A0 use this and have a flag in the page structure indicat=
ing
>> whether that page has been freed/used or not.
> Not sure what do you mean. The refcount does this right?
I meant a flag using which I could determine whether a PFN has been
already freed by the host or not. This is to avoid repetitive free.
>
>> Though I am not sure if
>> this will be acceptable upstream.
>> Second is to find another place to invoke guest_free_page() post buddy=

>> merging.
> Might be easier.
>
>>>>>> ---
>>>>>>  arch/x86/include/asm/page.h |   12 ++++++++++++
>>>>>>  arch/x86/kernel/kvm.c       |   28 ++++++++++++++++++++++++++++
>>>>>>  include/linux/gfp.h         |    4 ++++
>>>>>>  mm/page_alloc.c             |    2 ++
>>>>>>  4 files changed, 46 insertions(+)
>>>>>>
>>>>>> diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/pa=
ge.h
>>>>>> index 4487ad7a3385..9540a97c9997 100644
>>>>>> --- a/arch/x86/include/asm/page.h
>>>>>> +++ b/arch/x86/include/asm/page.h
>>>>>> @@ -29,6 +29,18 @@ 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);
>>>>>>  }
>>>>>> +
>>>>>> +struct zone;
>>>>>> +
>>>>>> +#define HAVE_ARCH_MERGE_PAGE
>>>>>> +void __arch_merge_page(struct zone *zone, struct page *page,
>>>>>> +		       unsigned int order);
>>>>>> +static inline void arch_merge_page(struct zone *zone, struct page=
 *page,
>>>>>> +				   unsigned int order)
>>>>>> +{
>>>>>> +	if (static_branch_unlikely(&pv_free_page_hint_enabled))
>>>>>> +		__arch_merge_page(zone, page, order);
>>>>>> +}
>>>>>>  #endif
>>>>>> =20
>>>>>>  #include <linux/range.h>
>>>>>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>>>>>> index 09c91641c36c..957bb4f427bb 100644
>>>>>> --- a/arch/x86/kernel/kvm.c
>>>>>> +++ b/arch/x86/kernel/kvm.c
>>>>>> @@ -785,6 +785,34 @@ void __arch_free_page(struct page *page, unsi=
gned int order)
>>>>>>  		       PAGE_SIZE << order);
>>>>>>  }
>>>>>> =20
>>>>>> +void __arch_merge_page(struct zone *zone, struct page *page,
>>>>>> +		       unsigned int order)
>>>>>> +{
>>>>>> +	/*
>>>>>> +	 * The merging logic has merged a set of buddies up to the
>>>>>> +	 * KVM_PV_UNUSED_PAGE_HINT_MIN_ORDER. Since that is the case, ta=
ke
>>>>>> +	 * advantage of this moment to notify the hypervisor of the free=

>>>>>> +	 * memory.
>>>>>> +	 */
>>>>>> +	if (order !=3D KVM_PV_UNUSED_PAGE_HINT_MIN_ORDER)
>>>>>> +		return;
>>>>>> +
>>>>>> +	/*
>>>>>> +	 * Drop zone lock while processing the hypercall. This
>>>>>> +	 * should be safe as the page has not yet been added
>>>>>> +	 * to the buddy list as of yet and all the pages that
>>>>>> +	 * were merged have had their buddy/guard flags cleared
>>>>>> +	 * and their order reset to 0.
>>>>>> +	 */
>>>>>> +	spin_unlock(&zone->lock);
>>>>>> +
>>>>>> +	kvm_hypercall2(KVM_HC_UNUSED_PAGE_HINT, page_to_phys(page),
>>>>>> +		       PAGE_SIZE << order);
>>>>>> +
>>>>>> +	/* reacquire lock and resume freeing memory */
>>>>>> +	spin_lock(&zone->lock);
>>>>>> +}
>>>>>> +
>>>>>>  #ifdef CONFIG_PARAVIRT_SPINLOCKS
>>>>>> =20
>>>>>>  /* Kick a cpu by its apicid. Used to wake up a halted vcpu */
>>>>>> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
>>>>>> index fdab7de7490d..4746d5560193 100644
>>>>>> --- a/include/linux/gfp.h
>>>>>> +++ b/include/linux/gfp.h
>>>>>> @@ -459,6 +459,10 @@ static inline struct zonelist *node_zonelist(=
int nid, gfp_t flags)
>>>>>>  #ifndef HAVE_ARCH_FREE_PAGE
>>>>>>  static inline void arch_free_page(struct page *page, int order) {=
 }
>>>>>>  #endif
>>>>>> +#ifndef HAVE_ARCH_MERGE_PAGE
>>>>>> +static inline void
>>>>>> +arch_merge_page(struct zone *zone, struct page *page, int order) =
{ }
>>>>>> +#endif
>>>>>>  #ifndef HAVE_ARCH_ALLOC_PAGE
>>>>>>  static inline void arch_alloc_page(struct page *page, int order) =
{ }
>>>>>>  #endif
>>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>>>> index c954f8c1fbc4..7a1309b0b7c5 100644
>>>>>> --- a/mm/page_alloc.c
>>>>>> +++ b/mm/page_alloc.c
>>>>>> @@ -913,6 +913,8 @@ static inline void __free_one_page(struct page=
 *page,
>>>>>>  		page =3D page + (combined_pfn - pfn);
>>>>>>  		pfn =3D combined_pfn;
>>>>>>  		order++;
>>>>>> +
>>>>>> +		arch_merge_page(zone, page, order);
>>>>>>  	}
>>>>>>  	if (max_order < MAX_ORDER) {
>>>>>>  		/* If we are here, it means order is >=3D pageblock_order.
>>>> --=20
>>>> Regards
>>>> Nitesh
>>>>
>>>
>> --=20
>> Regards
>> Nitesh
>>
>
>
--=20
Regards
Nitesh


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