On Thu, Feb 06, 2020 at 11:51:16AM -0500, Peter Xu wrote: > On Thu, Feb 06, 2020 at 08:28:18AM -0800, Sean Christopherson wrote: > > 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. > > Ah I see, thanks. Another alternative is we move the "old = *slot" > copy into kvm_delete_memslot(), which could be even clearer imo. The copy is also needed in __kvm_set_memory_region() for the MOVE case. > However I'm not sure whether it's a good idea to drop the test-by for > this. Considering that comment change should not affect it, would you > mind enrich the comment into something like this (or anything better)? > > /* > * Make a full copy of the old memslot, the pointer will become stale > * when the memslots are re-sorted by update_memslots() in > * kvm_delete_memslot(), while to make the kvm_free_memslot() work as > * expected later on, we still need the cached memory slot. > */ As above, it's more subtle than just the kvm_delete_memslot() case. /* * Make a full copy of the old memslot, the pointer will become stale * when the memslots are re-sorted by update_memslots() when deleting * or moving a memslot, and additional modifications to the old memslot * need to be made after calling update_memslots(). */
