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dm-ioctl
During initialisation, Device Mapper sets up a misc control device (/dev/mapper/control) for root-only ioctl communication.
This has major number 10 (misc) and a dynamically-allocated minor number (normally 63).
We ignore the 'size' part of the ioctl definition (_IOC_SIZE).
Requests are of variable size – userspace has to provide sufficient space for the answer, otherwise the answer is truncated, DM_BUFFER_FULL_FLAG gets set and userspace should retry with a larger buffer. libdevmapper handles this transparently, doubling the buffer size and retrying.