if PAGE_SIZE >= SMK_LOADSIZE all legitimate requests can be made
using PAGE_SIZE as a limit. Your example with 19990 spaces before
the data demonstrates that the interface is inadequately documented.
Tizen and Automotive Grade Linux are going to be fine with a PAGE_SIZE
limit. The best way to address this concern is to go ahead with the
PAGE_SIZE limit and create ABI documents for the smackfs interfaces.
I will take your patch for the former and create a patch for the latter.

Please let me know if there is anything else required for this patch.
AFAIU, we agreed with PAGE_SIZE as the limit.