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

[Aaron Lu]

On 2019/2/11 23:58, Alexander Duyck wrote:
> On Mon, 2019-02-11 at 14:40 +0800, Aaron Lu wrote:
>> On 2019/2/5 2:15, Alexander Duyck wrote:
>>> From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxxxxxxx>
>>> ---
>>>  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);
>>
>> Not a proper place AFAICS.
>>
>> Assume we have an order-8 page being sent here for merge and its order-8
>> buddy is also free, then order++ became 9 and arch_merge_page() will do
>> the hint to host on this page as an order-9 page, no problem so far.
>> Then the next round, assume the now order-9 page's buddy is also free,
>> order++ will become 10 and arch_merge_page() will again hint to host on
>> this page as an order-10 page. The first hint to host became redundant.
> 
> Actually the problem is even worse the other way around. My concern was
> pages being incrementally freed.
> 
> With this setup I can catch when we have crossed the threshold from
> order 8 to 9, and specifically for that case provide the hint. This
> allows me to ignore orders above and below 9.

OK, I see, you are now only hinting for pages with order 9, not above.

> If I move the hint to the spot after the merging I have no way of
> telling if I have hinted the page as a lower order or not. As such I
> will hint if it is merged up to orders 9 or greater. So for example if
> it merges up to order 9 and stops there then done_merging will report
> an order 9 page, then if another page is freed and merged with this up
> to order 10 you would be hinting on order 10. By placing the function
> here I can guarantee that no more than 1 hint is provided per 2MB page.

So what's the downside of hinting the page as order-10 after merge
compared to as order-9 before the merge? I can see the same physical
range can be hinted multiple times, but the total hint number is the
same: both are 2 - in your current implementation, we hint twice for
each of the 2 order-9 pages; alternatively, we can provide hint for one
order-9 page and the merged order-10 page. I think the cost of
hypercalls are the same? Is it that we want to ease the host side
madvise(DONTNEED) since we can avoid operating the same range multiple
times?

The reason I asked is, if we can move the arch_merge_page() after
done_merging tag, we can theoretically make fewer function calls on free
path for the guest. Maybe not a big deal, I don't know...

>> I think the proper place is after the done_merging tag.
>>
>> BTW, with arch_merge_page() at the proper place, I don't think patch3/4
>> is necessary - any freed page will go through merge anyway, we won't
>> lose any hint opportunity. Or do I miss anything?
> 
> You can refer to my comment above. What I want to avoid is us hinting a
> page multiple times if we aren't using MAX_ORDER - 1 as the limit. What

Yeah that's a good point. But is this going to happen?

> I am avoiding by placing this where I did is us doing a hint on orders
> greater than our target hint order. So with this way I only perform one
> hint per 2MB page, otherwise I would be performing multiple hints per
> 2MB page as every order above that would also trigger hints.
>