Add notes for section 13.3.

13.md

@@ -120,10 +120,38 @@ reset:
   this, we want to avoid killing a process as much as we can which is why there
   are checks in the first instance.
 
+## 13.3 Selecting a Process
+
+* [select_bad_process()][select_bad_process] determines the process to kill by
+  stepping through each running task and calculating how suitable it is for
+  killing via the function [badness()][badness], which determines this via:
+
+```
+badness_for_task = total_vm_for_task/(cpu_time_in_seconds^0.5 * cpu_time_in_minutes^0.25)
+```
+
+* The square roots are approximated by [int_sqrt()][int_sqrt].
+
+* This has been chosen to prefer a process that is using a large amount of
+  memory but is not that long-lived. Long-lived processes are unlikely to be the
+  cause of memory shortage.
+
+* If the process is a root process or has `CAP_SYS_ADMIN` capabilities, the
+  points are divided by 4 since it is assumed that privileged processes are
+  well-behaved.
+
+* Similarly, if the process has `CAP_SYS_RAWIO` capabilities (access to raw
+  device), the points are further divided by 4 because it's not a good idea to
+  kill a process that has direct access to hardware.
+
 [brk]:http://man7.org/linux/man-pages/man2/brk.2.html
 [mremap]:http://man7.org/linux/man-pages/man2/mremap.2.html
 [out_of_memory]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/oom_kill.c#L202
 [vm_enough_memory]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/mmap.c#L53
 [swapper_space]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/swap_state.c#L39
 [dentry]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/include/linux/dcache.h#L67
 [inode]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/include/linux/fs.h#L438
+
+[select_bad_process]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/oom_kill.c#L121
+[badness]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/oom_kill.c#L40
+[int_sqrt]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/oom_kill.c#L26