While on a 32-bit arch the tail pointer is 32 bits, the struct itself contains (and has always contained) a 64-bit value 'quota' with a forced 64-bit alignment which forces 64-bit alignment of the entire struct and thus also forces tail padding to make it a multiple of 64-bits. ie. size doesn't change - even on 32 bits. Chenbo was testing compatibility on both 32 and 64 bit x86 VMs.
