Hi Vishal, On Wed, 5 Feb 2025 at 00:42, Vishal Annapurve <vannapurve@xxxxxxxxxx> wrote: > > On Fri, Jan 17, 2025 at 8:30 AM Fuad Tabba <tabba@xxxxxxxxxx> wrote: > > > > Before transitioning a guest_memfd folio to unshared, thereby > > disallowing access by the host and allowing the hypervisor to > > transition its view of the guest page as private, we need to be > > sure that the host doesn't have any references to the folio. > > > > This patch introduces a new type for guest_memfd folios, and uses > > that to register a callback that informs the guest_memfd > > subsystem when the last reference is dropped, therefore knowing > > that the host doesn't have any remaining references. > > > > Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx> > > --- > > The function kvm_slot_gmem_register_callback() isn't used in this > > series. It will be used later in code that performs unsharing of > > memory. I have tested it with pKVM, based on downstream code [*]. > > It's included in this RFC since it demonstrates the plan to > > handle unsharing of private folios. > > > > [*] https://android-kvm.googlesource.com/linux/+/refs/heads/tabba/guestmem-6.13-v5-pkvm > > Should the invocation of kvm_slot_gmem_register_callback() happen in > the same critical block as setting the guest memfd range mappability > to NONE, otherwise conversion/truncation could race with registration > of callback? I don't think it needs to, at least not as far potencial races are concerned. First because kvm_slot_gmem_register_callback() grabs the mapping's invalidate_lock as well as the folio lock, and gmem_clear_mappable() grabs the mapping lock and the folio lock if a folio has been allocated before. Second, __gmem_register_callback() checks before returning whether all references have been dropped, and adjusts the mappability/shareability if needed. Cheers, /fuad
