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

On 24/06/21 14:57, Nicholas Piggin wrote:

quoted

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".

quoted

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