One page or 4% of physical memory, whichever is greater.
Number of bytes in one page (8192 for UltraSPARCTM systems, 4096 for all others) to 25% of the available kernel memory at the time TMPFS was first used.
Increase if the following message is displayed on the console or written in the messages file.
tmp_memalloc: tmpfs over memory limit
The current amount of memory used by TMPFS for its data structures is held in the tmp_kmemspace field, which can be examined with a kernel debugger.