mmap implementation advice needed.

[Anders Magnusson <ragge%ludd.ltu.se@localhost>]

Hi tech-kern,

I'm trying to solve PR#28379 and ran into a problem and I don't really 
understand how it is supposed to work:
If a process tries to mmap for example a file with a length of just over 
1GB it will always succeed as I understand the code, but that may not be 
true depending on the underlying hardware, and I cannot find any way to 
control this from the MD code...?

On vax, for example, large mmap's cannot be done due to hardware 
constraints.
In the above example it will cause the mmap() to succeed, but when 
touching the pages it will hang forever since there will never be 
available pte's.

So, any advice how a max size of allowed mmap'able memory be controlled?

Notes about vax memory management if someone is wondering:
- 2 areas (P0 and P1) of size 1G each, P0 grows from bottom, P1 grows 
from top (intended for stack).
- The PTEs for KVM must be in contiguous physical memory, hence the 
allocation for one process with all of P0 and P1 mapped takes 128k.
- Vax uses VM_MAP_TOPDOWN so that not too much of KVM space is needed 
for mmap.

-- Ragge