uvm page coloring for cache aliasing

[coypu%sdf.org@localhost]

Hi folks,

as I understand, a reason to have page coloring is extra broken MIPS
hardware which has cache aliasing issues unless a large page size
is used. picking the same color avoids aliasing.

now, looking at uvm_pagealloc_pgfl:

do {
	/* trying to find pages in color.. */
	/* goto success */

	color = (color + 1) & uvmexp.colormask;
 } while (color != trycolor);

which means that if we fail to find a page in the requested color, we'll
try another color. I think this might end up inducing cache aliasing
issues, and we should instead fail for this case.

thoughts?