Charlie Jenkins [off-list ref] writes:

Some applications rely on placing data in free bits addresses allocated
by mmap. Various architectures (eg. x86, arm64, powerpc) restrict the
address returned by mmap to be less than the 48-bit address space,
unless the hint address uses more than 47 bits (the 48th bit is reserved
for the kernel address space).

To make this behavior explicit and more versatile across all
architectures, define a mmap flag that allows users to define an
arbitrary upper limit on addresses returned by mmap.

Signed-off-by: Charlie Jenkins <redacted>
---
 include/uapi/asm-generic/mman-common.h       | 1 +
 tools/include/uapi/asm-generic/mman-common.h | 1 +

  
You're not meant to update the headers in tools/ directly. There's a
mail somewhere from acme somewhere describing the proper process, but
the tldr is leave it up to him.

IMHO the API would be clearer if this actually forced the address to be
below the hint. That's what the flag name implies after all.

It would also mean the application doesn't need to take into account the
length of the mapping when passing the hint.

cheers