Since 2.6.5, Documentation/devices.txt says
0 Unnamed devices (e.g. non-device mounts)
0 = reserved as null device number
See block major 144, 145, 146 for expansion areas.
144 block Expansion Area #1 for more non-device (e.g. NFS) mounts
0 = mounted device 256
255 = mounted device 511
145 block Expansion Area #2 for more non-device (e.g. NFS) mounts
0 = mounted device 512
255 = mounted device 767
146 block Expansion Area #3 for more non-device (e.g. NFS) mounts
0 = mounted device 768
255 = mounted device 1023
In 2.6 kernels, device minor number field has 20 bits ( #define MINORBITS 20 ).
If all minor numbers for major == 0 (i.e. 1048576 entries) are already in use,
accepting 256 minor numbers for each 3 major numbers (i.e. 768 entries) sounds
helpless.
Are major == 144, 145, 146 really used? If yes, when are these used?
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