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