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
