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Date:   Mon, 11 Feb 2019 12:41:08 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Nitesh Narayan Lal <nitesh@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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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 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.

Not sure what do you mean. The refcount does this right?

> 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/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
>