The /proc/PID/clear_refs is used to reset the PG_Referenced and ACCESSED/YOUNG bits on both physical and virtual pages associated with a process.

To clear the bits for all the pages associated with the process:

echo 1 > /proc/PID/clear_refs

To clear the bits for the anonymous pages associated with the process:

echo 2 > /proc/PID/clear_refs

To clear the bits for the file mapped pages associated with the process:

echo 3 > /proc/PID/clear_refs

Any other value written to /proc/PID/clear_refs will have no effect.