I had an idea that should work well with Gerard's patch for buffer head
allocation. As long as you're distinguishing page IO from other
operations, why not reserve some number (say 50 or 100) buffer heads for
page IO? So if the number of free buffer heads is less than the
reserve, non-page IO requests would have to try to allocate a new one,
but page IO could still use the free heads.
This would be easy to implement, and should improve system stability by
ensuring that paging IO doesn't suffer when memory gets very low.
-Bill