Linux VirtIO Memory Prepares For "Big Block Mode"
Right now the virtio-mem driver only supports device block sizes that max out at the size of a single Linux memory block. The "Big Block Mode" addresses that and allows any device block size for any Linux virtual machine.
Big Block Mode is also important for Red Hat's work on VFIO support in QEMU. Ultimately this virtio-mem work allows for easily creating blocks in the gigabyte range and can be hot added/removed. A Big Block "BB" spans multiple Linux memory blocks and is added/removed at Big Block granularity to the memory management code. The sub-block mode is still supported with the mode being determined by the Linux memory block size and device blok size.
If this interests you, learn more about the Big Block Mode via this patch series.