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

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Message-ID: <54226CEB.9080504@redhat.com>
Date:	Wed, 24 Sep 2014 09:04:11 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Andres Lagar-Cavilla <andreslc@...gle.com>
CC:	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
Subject: Re: [PATCH v4] kvm: Fix page ageing bugs

Il 24/09/2014 04:27, Wanpeng Li ha scritto:
> Hi Andres,
> On Mon, Sep 22, 2014 at 02:54:42PM -0700, Andres Lagar-Cavilla wrote:
>> 1. We were calling clear_flush_young_notify in unmap_one, but we are
>> within an mmu notifier invalidate range scope. The spte exists no more
>> (due to range_start) and the accessed bit info has already been
>> propagated (due to kvm_pfn_set_accessed). Simply call
>> clear_flush_young.
>>
>> 2. We clear_flush_young on a primary MMU PMD, but this may be mapped
>> as a collection of PTEs by the secondary MMU (e.g. during log-dirty).
>> This required expanding the interface of the clear_flush_young mmu
>> notifier, so a lot of code has been trivially touched.
>>
>> 3. In the absence of shadow_accessed_mask (e.g. EPT A bit), we emulate
>> the access bit by blowing the spte. This requires proper synchronizing
>> with MMU notifier consumers, like every other removal of spte's does.
>>
> [...]
>> ---
>> +	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);
> 
> IIUC, all the rmapps in this for loop are against the same gfn which
> results in the above trace point dump the message duplicated.

You're right; Andres's patch "[PATCH] kvm/x86/mmu: Pass gfn and level to
rmapp callback" helps avoiding that.

Paolo
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