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From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 0/6] KVM: Remove uses of struct page from x86 and arm64
 MMU
Message-ID: <81d99029-ec40-19c5-5647-20607d78dab0@redhat.com>
Date:   Thu, 24 Jun 2021 17:35:25 +0200
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On 24/06/21 14:57, Nicholas Piggin wrote:
> KVM: Fix page ref underflow for regions with valid but non-refcounted pages

It doesn't really fix the underflow, it disallows mapping them in the 
first place.  Since in principle things can break, I'd rather be 
explicit, so let's go with "KVM: do not allow mapping valid but 
non-reference-counted pages".

> It's possible to create a region which maps valid but non-refcounted
> pages (e.g., tail pages of non-compound higher order allocations). These
> host pages can then be returned by gfn_to_page, gfn_to_pfn, etc., family
> of APIs, which take a reference to the page, which takes it from 0 to 1.
> When the reference is dropped, this will free the page incorrectly.
> 
> Fix this by only taking a reference on the page if it was non-zero,

s/on the page/on valid pages/ (makes clear that invalid pages are fine 
without refcounting).

Thank you *so* much, I'm awful at Linux mm.

Paolo

> which indicates it is participating in normal refcounting (and can be
> released with put_page).
> 
> Signed-off-by: Nicholas Piggin<npiggin@gmail.com>