On Thu, Feb 06, 2020 at 11:14:15AM -0500, Peter Xu wrote: > On Tue, Jan 21, 2020 at 02:31:50PM -0800, Sean Christopherson wrote: > > Move memslot deletion into its own routine so that the success path for > > other memslot updates does not need to use kvm_free_memslot(), i.e. can > > explicitly destroy the dirty bitmap when necessary. This paves the way > > for dropping @dont from kvm_free_memslot(), i.e. all callers now pass > > NULL for @dont. > > > > Add a comment above the code to make a copy of the existing memslot > > prior to deletion, it is not at all obvious that the pointer will become > > stale during sorting and/or installation of new memslots. > > Could you help explain a bit on this explicit comment? I can follow > up with the patch itself which looks all correct to me, but I failed > to catch what this extra comment wants to emphasize... It's tempting to write the code like this (I know, because I did it): if (!mem->memory_size) return kvm_delete_memslot(kvm, mem, slot, as_id); new = *slot; Where @slot is a pointer to the memslot to be deleted. At first, second, and third glances, this seems perfectly sane. The issue is that slot was pulled from struct kvm_memslots.memslots, e.g. slot = &slots->memslots[index]; Note that slots->memslots holds actual "struct kvm_memory_slot" objects, not pointers to slots. When update_memslots() sorts the slots, it swaps the actual slot objects, not pointers. I.e. after update_memslots(), even though @slot points at the same address, it's could be pointing at a different slot. As a result kvm_free_memslot() in kvm_delete_memslot() will free the dirty page info and arch-specific points for some random slot, not the intended slot, and will set npages=0 for that random slot. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
