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

Nicholas Piggin
2021-06-25T00:20:40Z

Excerpts from Paolo Bonzini's message of June 25, 2021 1:35 am:
> 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).

That seems okay, you can adjust the title or changelog as you like.

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

Glad to help. Easy to see why you were taking this approach because the
API really does need to be improved and even a pretty intwined with mm
subsystem like KVM shouldn't _really_ be doing this kind of trick (and
it should go away when old API is removed).

Thanks,
Nick