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I couldn't find a definitive authoritative documentation anywhere but this seems pretty good:

Mel Gorman: _Understanding the Linux Virtual Memory Manager_, [Section 13.4](https://www.kernel.org/doc/gorman/html/understand/understand016.html)

> Once a task is selected, the list is walked again and each process that shares the same mm_struct as the selected process (i.e. they are threads) is sent a signal. If the process has CAP_SYS_RAWIO capabilities, a SIGTERM is sent to give the process a chance of exiting cleanly, otherwise a SIGKILL is sent.