7b85f59183
pmap_ts_referenced() is not always appropriate for checking whether or not pages have been referenced because it clears any reference bits that it encounters. For example, in mincore(), clearing the reference bits has two negative consequences. First, it throws off the activity count calculations performed by the page daemon. Specifically, a page on which mincore() has called pmap_ts_referenced() looks less active to the page daemon than it should. Consequently, the page could be deactivated prematurely by the page daemon. Arguably, this problem could be fixed by having mincore() duplicate the activity count calculation on the page. However, there is a second problem for which that is not a solution. In order to clear a reference on a 4KB page, it may be necessary to demote a 2/4MB page mapping. Thus, a mincore() by one process can have the side effect of demoting a superpage mapping within another process! |
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