Re: [RFC PATCH 4/4] mm: Add merge page notifier

[Nitesh Narayan Lal <nitesh@redhat.com>]

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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 hints 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 then
>>>> huge TLB in size and aggregated with buddies later.
>>>>
>>>> To address that I am adding a new call arch_merge_page which is called
>>>> after __free_one_page has merged a pair of pages to create a higher order
>>>> page. By doing this I am able to fill the gap and provide full coverage 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 report
>> 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  use this and have a flag in the page structure indicating
whether that page has been freed/used or not. 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.

>
>>>
>>>> ---
>>>>  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/page.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
>>>>  
>>>>  #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, unsigned int order)
>>>>  		       PAGE_SIZE << order);
>>>>  }
>>>>  
>>>> +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, take
>>>> +	 * advantage of this moment to notify the hypervisor of the free
>>>> +	 * memory.
>>>> +	 */
>>>> +	if (order != 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
>>>>  
>>>>  /* 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 = page + (combined_pfn - pfn);
>>>>  		pfn = combined_pfn;
>>>>  		order++;
>>>> +
>>>> +		arch_merge_page(zone, page, order);
>>>>  	}
>>>>  	if (max_order < MAX_ORDER) {
>>>>  		/* If we are here, it means order is >= pageblock_order.
>> -- 
>> Regards
>> Nitesh
>>
>
>
-- 
Regards
Nitesh


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