I know that the apparent 1.5L change makes sense that
page_idle_zero and UBC seem to be mutually exclusive --
it's self-defeating to have usable buffer cache pages to
be zorched just because the machine is idle -- but would
it be reasonable to have a target of a few % of all pages
sitting zero so that if something (fs i/o or whatnot)
wants a burst of zero-filled pages the zero-fill penalty
has been at least partially prepaid during idle moments?

It seems that the worst case would be that one zeros a
page containing file-system data that one re-reads during
the next burst of system activity, so perhaps this means
"fine-tuning required".

(Come to think of it, should "fs read faults" eat spare
zero-filled pages if we are below a threshold, or should
those pages be set aside for more CPU-driven activities
which need fresh pages?)

        Sean.