"Aneesh Kumar K.V" [off-list ref] writes:

quoted

If it is decided to keep these kind of heuristics, can we get just a
small but reasonably precise description of each change to the
interface and ways for using the new functionality, such that would be
suitable for the man page? I couldn't fix powerpc because nothing
matches and even Aneesh and you differ on some details (MAP_FIXED
behaviour).

I would consider MAP_FIXED as my mistake. We never discussed this 
explicitly and I kind of assumed it to behave the same way. ie, we 
search in lower address space (128TB) if the hint addr is below 128TB.

IIUC we agree on the below.

1) MAP_FIXED allow the addr to be used, even if hint addr is below 128TB 
but hint_addr + len is > 128TB.

So:
  mmap(0x7ffffffff000, 0x2000, ..., MAP_FIXED ...) = 0x7ffffffff000

2) For everything else we search in < 128TB space if hint addr is below 
128TB

  mmap((x < 128T), 0x1000, ...) = (y < 128T)
  ...
  mmap(0x7ffffffff000, 0x1000, ...) = 0x7ffffffff000
  mmap(0x800000000000, 0x1000, ...) = 0x800000000000
  ...
  mmap((x >= 128T), 0x1000, ...) = (y >= 128T)

3) We don't switch to large address space if hint_addr + len > 128TB. 
The decision to switch to large address space is primarily based on hint 
addr

But does the mmap succeed in that case or not?

ie:  mmap(0x7ffffffff000, 0x2000, ...) = ?

cheers