On 06.02.20 10:27, Michael S. Tsirkin wrote: > On Mon, Feb 03, 2020 at 07:31:12PM +0100, David Hildenbrand 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/ > > How does this inteact with all the prealloc/mlock things designed > for realtime?
- Prealloc: we don't support resizable ram blocks with prealloc -- qemu_ram_alloc_from_ptr() is the only way to get "real" preallocated ram blocks from a pointer. They are never resizable. -- "prealloc" with memory backends (e.g., backends/hostmem.c): Memory backends don't support resizable ram blocks yet. I have patches to support that, and disallow prealloc for them. -- file-based ram blocks are not resizable - mlock -- os_mlock() does a mlockall(MCL_CURRENT | MCL_FUTURE) -- That should work just fine with ordinary mmap() invalidating old mmaps and creating new mmaps(). Just like when hotplugging/unplugging a DIMM. Resizing currently only happens during reboot/migration. If mlock_all would result in issues, we could fallback to old handling (if (enable_mlock) ...) - but I don't think this is necessary. So I don't see an issues with that :) Thanks! -- Thanks, David / dhildenb
