The vm_page_io_start function prepares the page for I/O by incrementing its busy flag by one.
The vm_page_io_finish function lowers the busy count on the page by one, if the resulting busy count is zero, a wakeup(9) will be issued if the page has been marked PG_WANTED. A page is typically marked PG_WANTED by a thread to register its interest in the page to either complete I/O or becoming available for general use.
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.An Alfred Perlstein Aq alfred@FreeBSD.org .