linux-kernel - Re: [PATCH 0/2] KVM: MMU: support VMAs that got remap_pfn

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Date:	Mon, 4 Jul 2016 10:14:56 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	Neo Jia <cjia@...dia.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Kirti Wankhede <kwankhede@...dia.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 0/2] KVM: MMU: support VMAs that got remap_pfn_range-ed



On 04/07/2016 09:59, Xiao Guangrong wrote:
> 
>> But apart from this, it's much more obvious to consider the refcount.
>> The x86 MMU code doesn't care if the page is reserved or not;
>> mmu_set_spte does a kvm_release_pfn_clean, hence it makes sense for
>> hva_to_pfn_remapped to try doing a get_page (via kvm_get_pfn) after
>> invoking the fault handler, just like the get_user_pages family of
>> function does.
> 
> Well,  it's little strange as you always try to get refcont
> for a PFNMAP region without MIXEDMAP which indicates all the memory
> in this region is no 'struct page' backend.

Fair enough, I can modify the comment.

	/*
	 * In case the VMA has VM_MIXEDMAP set, whoever called remap_pfn_range
	 * is also going to call e.g. unmap_mapping_range before the underlying
	 * non-reserved pages are freed, which will then call our MMU notifier.
	 * We still have to get a reference here to the page, because the callers
	 * of *hva_to_pfn* and *gfn_to_pfn* ultimately end up doing a
	 * kvm_release_pfn_clean on the returned pfn.  If the pfn is
	 * reserved, the kvm_get_pfn/kvm_release_pfn_clean pair will simply
	 * do nothing.
	 */

Paolo

> But it works as kvm_{get, release}_* have already been aware of
> reserved_pfn, so i am okay with it......