linux-kernel - Re: [PATCH v4] kvm: Fix page ageing bugs

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Date:	Tue, 23 Sep 2014 09:49:05 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Gleb Natapov <gleb@...nel.org>,
	Radim Krcmar <rkrcmar@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Peter Feiner <pfeiner@...gle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
CC:	Andres Lagar-Cavilla <andreslc@...ogle.com>
Subject: Re: [PATCH v4] kvm: Fix page ageing bugs

Il 22/09/2014 23:54, Andres Lagar-Cavilla ha scritto:
> @@ -1406,32 +1406,24 @@ static int kvm_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
>  	struct rmap_iterator uninitialized_var(iter);
>  	int young = 0;
>  
> -	/*
> -	 * In case of absence of EPT Access and Dirty Bits supports,
> -	 * emulate the accessed bit for EPT, by checking if this page has
> -	 * an EPT mapping, and clearing it if it does. On the next access,
> -	 * a new EPT mapping will be established.
> -	 * This has some overhead, but not as much as the cost of swapping
> -	 * out actively used pages or breaking up actively used hugepages.
> -	 */
> -	if (!shadow_accessed_mask) {
> -		young = kvm_unmap_rmapp(kvm, rmapp, slot, data);
> -		goto out;
> -	}
> +	BUG_ON(!shadow_accessed_mask);
>  
>  	for (sptep = rmap_get_first(*rmapp, &iter); sptep;
>  	     sptep = rmap_get_next(&iter)) {
> +		struct kvm_mmu_page *sp;
> +		gfn_t gfn;
>  		BUG_ON(!is_shadow_present_pte(*sptep));
> +		/* From spte to gfn. */
> +		sp = page_header(__pa(sptep));
> +		gfn = kvm_mmu_page_get_gfn(sp, sptep - sp->spt);
>  
>  		if (*sptep & shadow_accessed_mask) {
>  			young = 1;
>  			clear_bit((ffs(shadow_accessed_mask) - 1),
>  				 (unsigned long *)sptep);
>  		}
> +		trace_kvm_age_page(gfn, slot, young);

Yesterday I couldn't think of a way to avoid the
page_header/kvm_mmu_page_get_gfn on every iteration, but it's actually
not hard.  Instead of passing hva as datum, you can pass (unsigned long)
&start.  Then you can add PAGE_SIZE to it at the end of every call to
kvm_age_rmapp, and keep the old tracing logic.


Paolo
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