Re: [Intel-gfx] [PATCH 0/6] KVM: Remove uses of struct page from x86 and arm64 MMU

Paolo Bonzini
2021-06-24T15:35:36Z

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>