linux-kernel - Re: [RFC][PATCH 3/9] replace x86 kvm n_free_mmu_pages with n_used_mmu

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Date:	Wed, 16 Jun 2010 11:25:41 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC][PATCH 3/9] replace x86 kvm n_free_mmu_pages with
 n_used_mmu_pages

On Tue, Jun 15, 2010 at 06:55:22AM -0700, Dave Hansen wrote:
> 
> I think doing this makes the code much more readable.  That's
> borne out by the fact that this patch removes code.  "used"
> also happens to be the number that we need to return back to
> the slab code when our shrinker gets called.  Keeping this
> value as opposed to free makes the next patch simpler.
> 
> So, 'struct kvm' is kzalloc()'d.  'struct kvm_arch' is a
> structure member (and not a pointer) of 'struct kvm'.  That
> means they start out zeroed.  I _think_ they get initialized
> properly by kvm_mmu_change_mmu_pages().  But, that only happens
> via kvm ioctls.
> 
> I have a suspicion that they values are actually inconsistent
> until those ioctls get called; "free" and "alloc" are both zero.
> But, the VM can't really get run until these ioctl()s get called
> anyway.  There are also some checks for negative "used_pages"
> values which confused me.  It might all tie together.
> 
> Anyway, another benefit of storing 'used' intead of 'free' is
> that the values are consistent from the moment the structure is
> allocated: no negative "used" value.
> 
> Signed-off-by: Dave Hansen <dave@...ux.vnet.ibm.com>
> ---
> 
>  linux-2.6.git-dave/arch/x86/include/asm/kvm_host.h |    2 -
>  linux-2.6.git-dave/arch/x86/kvm/mmu.c              |   29 +++++++--------------
>  linux-2.6.git-dave/arch/x86/kvm/mmu.h              |    3 +-
>  3 files changed, 13 insertions(+), 21 deletions(-)
> 
> diff -puN arch/x86/include/asm/kvm_host.h~replace-free-with-used arch/x86/include/asm/kvm_host.h
> --- linux-2.6.git/arch/x86/include/asm/kvm_host.h~replace-free-with-used	2010-06-09 15:14:29.000000000 -0700
> +++ linux-2.6.git-dave/arch/x86/include/asm/kvm_host.h	2010-06-09 15:14:29.000000000 -0700
> @@ -380,7 +380,7 @@ struct kvm_mem_aliases {
>  struct kvm_arch {
>  	struct kvm_mem_aliases *aliases;
>  
> -	unsigned int n_free_mmu_pages;
> +	unsigned int n_used_mmu_pages;
>  	unsigned int n_requested_mmu_pages;
>  	unsigned int n_max_mmu_pages;
>  	atomic_t invlpg_counter;
> diff -puN arch/x86/kvm/mmu.c~replace-free-with-used arch/x86/kvm/mmu.c
> --- linux-2.6.git/arch/x86/kvm/mmu.c~replace-free-with-used	2010-06-09 15:14:29.000000000 -0700
> +++ linux-2.6.git-dave/arch/x86/kvm/mmu.c	2010-06-09 15:14:29.000000000 -0700
> @@ -898,7 +898,7 @@ static void kvm_mmu_free_page(struct kvm
>  	__free_page(virt_to_page(sp->spt));
>  	__free_page(virt_to_page(sp->gfns));
>  	kfree(sp);
> -	++kvm->arch.n_free_mmu_pages;
> +	--kvm->arch.n_used_mmu_pages;
>  }
>  
>  static unsigned kvm_page_table_hashfn(gfn_t gfn)
> @@ -919,7 +919,7 @@ static struct kvm_mmu_page *kvm_mmu_allo
>  	bitmap_zero(sp->slot_bitmap, KVM_MEMORY_SLOTS + KVM_PRIVATE_MEM_SLOTS);
>  	sp->multimapped = 0;
>  	sp->parent_pte = parent_pte;
> -	--vcpu->kvm->arch.n_free_mmu_pages;
> +	++vcpu->kvm->arch.n_used_mmu_pages;
>  	return sp;
>  }
>  
> @@ -1516,39 +1516,30 @@ static int kvm_mmu_zap_page(struct kvm *
>  
>  /*
>   * Changing the number of mmu pages allocated to the vm
> - * Note: if kvm_nr_mmu_pages is too small, you will get dead lock
> + * Note: if goal_nr_mmu_pages is too small, you will get dead lock
>   */
> -void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned int kvm_nr_mmu_pages)
> +void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned int goal_nr_mmu_pages)
>  {
> -	int used_pages;
> -
> -	used_pages = kvm->arch.n_max_mmu_pages - kvm_mmu_available_pages(kvm);
> -	used_pages = max(0, used_pages);
> -
>  	/*
>  	 * If we set the number of mmu pages to be smaller be than the
>  	 * number of actived pages , we must to free some mmu pages before we
>  	 * change the value
>  	 */
>  
> -	if (used_pages > kvm_nr_mmu_pages) {
> -		while (used_pages > kvm_nr_mmu_pages &&
> +	if (kvm->arch.n_used_mmu_pages > goal_nr_mmu_pages) {
> +		while (kvm->arch.n_used_mmu_pages > goal_nr_mmu_pages &&
>  			!list_empty(&kvm->arch.active_mmu_pages)) {
>  			struct kvm_mmu_page *page;
>  
>  			page = container_of(kvm->arch.active_mmu_pages.prev,
>  					    struct kvm_mmu_page, link);
> -			used_pages -= kvm_mmu_zap_page(kvm, page);
> -			used_pages--;
> +			kvm->arch.n_used_mmu_pages -= kvm_mmu_zap_page(kvm, page);
> +			kvm->arch.n_used_mmu_pages--;

kvm->arch.n_used_mmu_pages is deaccounted for in kvm_mmu_zap_page -> kvm_mmu_free_page
already.

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