* David Hildenbrand (da...@redhat.com) wrote: > > > > Am 06.02.2020 um 21:11 schrieb Dr. David Alan Gilbert <dgilb...@redhat.com>: > > > > * David Hildenbrand (da...@redhat.com) wrote: > >> We already allow resizable ram blocks for anonymous memory, however, they > >> are not actually resized. All memory is mmaped() R/W, including the memory > >> exceeding the used_length, up to the max_length. > >> > >> When resizing, effectively only the boundary is moved. Implement actually > >> resizable anonymous allocations and make use of them in resizable ram > >> blocks when possible. Memory exceeding the used_length will be > >> inaccessible. Especially ram block notifiers require care. > >> > >> Having actually resizable anonymous allocations (via mmap-hackery) allows > >> to reserve a big region in virtual address space and grow the > >> accessible/usable part on demand. Even if "/proc/sys/vm/overcommit_memory" > >> is set to "never" under Linux, huge reservations will succeed. If there is > >> not enough memory when resizing (to populate parts of the reserved region), > >> trying to resize will fail. Only the actually used size is reserved in the > >> OS. > >> > >> E.g., virtio-mem [1] wants to reserve big resizable memory regions and > >> grow the usable part on demand. I think this change is worth sending out > >> individually. Accompanied by a bunch of minor fixes and cleanups. > >> > >> [1] https://lore.kernel.org/kvm/20191212171137.13872-1-da...@redhat.com/ > > > > There's a few bits I've not understood from skimming the patches: > > > > Thanks for having a look! > > > a) Am I correct in thinking you PROT_NONE the extra space so you can > > gkrow/shrink it? > > Yes! > > > b) What does kvm see - does it have a slot for the whole space or for > > only the used space? > > Only the used space. Resizing triggers a resize of the memory region. That > triggers memory notifiers, which remove the old kvm memslot and re-add the > new kvm memslot. (That‘s existing handling, so nothing new). > > So KVM will not see PROT_NONE when creating a slot.
OK, that's easy then. > > I ask because we found with virtiofs/DAX experiments that on Power, > > kvm gets upset if you give it a mapping with PROT_NONE. > > (That maybe less of an issue if you change the mapping after the > > slot is created). > > That should work as expected. Resizing *while* kvm is running is tricky, but > that‘s not part of this series and a different story :) right now, resizing > is only valid on reboot/incoming migration. Hmm 'when' during an incoming migration; I ask because of userfaultfd setup for postcopy. Also note those things can combine - i.e. a reboot that happens during a migration (we've already got a pile of related bugs). > > > > c) It's interesting this is keyed off the RAMBlock notifiers - do > > memory_listener's on the address space the block is mapped into get > > triggered? I'm wondering how vhost (and vhost-user) in particular > > see this. > > Yes, memory listeners get triggered. Old region is removed, new one is added. > Nothing changed on that front. > > The issue with ram block notifiers is that they did not do a „remove old, add > new“ on resizes. They only added the full ram block. Bad. E.g., vfio wants to > pin all memory - which would fail on PROT_NONE. > > E.g., for HAX, there is no kernel ioctl to remove a ram block ... for SEV > there is, but I am not sure about the implications when converting back and > forth between encrypted/unencrypted. So SEV and HAX require legacy handling. I guess for a memory listener it just sees a new layout after the commit and then can figure out what changed. Dave > Cheers! > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
