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Date:   Mon, 11 Feb 2019 09:17:18 -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 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?

> >
> >
> >> ---
> >>  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
>