From: Matthew Dobson <colpatch@us.ibm.com>

This resurrects the old /proc/sys/vm/free_pages functionality: the ability to
tell page reclaim how much free memory to maintain.

This may be needed for specialised networking applications, and it provides
an interesting way to stress the kernel: set it very low so atomic
allocations can easily fail.

Also, a 16G ppc64 box currently cruises along at 1M free memory, which is
surely too little to supporthigh-speed networking.  We have not changed that
setting here, but it is now possible to do so.

The patch also reduces the amount of free memory which the VM will maintain
in ZONE_HIGHMEM, as it is almost always wasted memory.