what is ~1MB of memory allocated at fffea000-fffff000 on ppc64?

[Paul Mackerras]

Chris Friesen writes:

> For some background, we're running an emulator that uses a null pointer 
> value of 0xffff0000 and we want any accesses to that address to trap.
> 
> Do you anticipate any issues with the following change?
> 
> -#define TASK_SIZE_USER32 (0x0000000100000000UL - (1*PAGE_SIZE))
> +#define TASK_SIZE_USER32 (0x00000000FFFF0000UL - (1*PAGE_SIZE))

Can you fix this in userspace instead by moving the stack down below
0xffff0000 and then doing munmap(0xffff0000, 0x1000) ?

Paul.