Prior to r254304, a separate function, vm_pageout_page_stats(), was used
to periodically update the reference status of the active pages. This
function was called, instead of vm_pageout_scan(), when memory was not
scarce. The objective was to provide up to date reference status for
active pages in case memory did become scarce and active pages needed to
be deactivated.
The active page queue scan performed by vm_pageout_page_stats() was
virtually identical to that performed by vm_pageout_scan(), and so r254304
eliminated vm_pageout_page_stats(). Instead, vm_pageout_scan() is
called with the parameter "pass" set to zero. The intention was that when
pass is zero, vm_pageout_scan() would only scan the active queue.
However, the variable page_shortage can still be greater than zero when
memory is not scarce and vm_pageout_scan() is called with pass equal to
zero. Consequently, the inactive queue may be scanned and dirty pages
laundered even though that was not intended by r254304. This revision
fixes that.
ab793415d0