linux-kernel - Re: [PATCH v5 1/4] KVM: mmu: introduce new gfn_to_pfn

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Date:   Thu, 30 Dec 2021 19:26:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Stevens <stevensd@...omium.org>
Cc:     Marc Zyngier <maz@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Will Deacon <will@...nel.org>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v5 1/4] KVM: mmu: introduce new gfn_to_pfn_page functions

On Mon, Nov 29, 2021, David Stevens wrote:
> +static kvm_pfn_t ensure_pfn_ref(struct page *page, kvm_pfn_t pfn)

"ensure" is rather misleading as that implies this is _just_ an assertion, but
that's not true since it elevates the refcount.  Maybe kvm_try_get_page_ref()?

> +{
> +	if (page || is_error_pfn(pfn))

A comment above here would be very helpful.  It's easy to overlook the "page"
check and think that KVM is double-counting pages.  E.g.

	/* If @page is valid, KVM already has a reference to the pfn/page. */

That would tie in nicely with the kvm_try_get_page_ref() name too.

> +		return pfn;
> +
> +	/*
> +	 * If we're here, a pfn resolved by hva_to_pfn_remapped is
> +	 * going to be returned to something that ultimately calls
> +	 * kvm_release_pfn_clean, so the refcount needs to be bumped if
> +	 * the pfn isn't a reserved pfn.
> +	 *
> +	 * Whoever called remap_pfn_range is also going to call e.g.
> +	 * unmap_mapping_range before the underlying pages are freed,
> +	 * causing a call to our MMU notifier.
> +	 *
> +	 * Certain IO or PFNMAP mappings can be backed with valid
> +	 * struct pages, but be allocated without refcounting e.g.,
> +	 * tail pages of non-compound higher order allocations, which
> +	 * would then underflow the refcount when the caller does the
> +	 * required put_page. Don't allow those pages here.
> +	 */
> +	if (kvm_is_reserved_pfn(pfn) ||
> +	    get_page_unless_zero(pfn_to_page(pfn)))
> +		return pfn;
> +
> +	return KVM_PFN_ERR_FAULT;
> +}