PATCH: Root reservations for strict overcommit

[Alan Cox <alan@redhat.com>]

This was on my TODO list for a while and it turns out someone already fixed the
armwaving overcommit mode for the same problem. It is easy to get into a
situation where you have no overcommit and nothing can be done because there is
no memory to clean up the stable but non-useful state of the machine.

The fix is trivial and duplicated from the armwaving overcommit code path.
The last 3% of the memory can be claimed by root processes only. It isn't a
cure but it does seem to solve the real world problems - at least providing
you have enough memory for 3% to be useful 8).

--- security/commoncap.c~	2004-08-31 15:27:46.777504736 +0100
+++ security/commoncap.c	2004-08-31 15:27:46.778504584 +0100
@@ -357,6 +357,11 @@
 
 	allowed = (totalram_pages - hugetlb_total_pages())
 	       	* sysctl_overcommit_ratio / 100;
+	/*
+	 * Leave the last 3% for root
+	 */
+	if (!capable(CAP_SYS_ADMIN))
+		allowed -= allowed / 32;
 	allowed += total_swap_pages;
 
 	if (atomic_read(&vm_committed_space) < allowed)