Alan made overcommit mode 2 and it doesnt work at all.  A process passing
the limit often does so at a moment of stack extension, and is killed by a
segfault, not better than being OOM-killed.

Another problem is that close to the edge no other processes can be
started, so that a sysadmin has problems logging in and investigating.

Below a patch that does 3 things:

(1) It reserves a reasonable amount of virtual stack space (amount
    randomly chosen, no guarantees given) when the process is started, so
    that the common utilities will not be killed by segfault on stack
    extension.

(2) It reserves a reasonable amount of virtual memory for root, so that
    root can do things when the system is out-of-memory

(3) It limits a single process to 97% of what is left, so that also an
    ordinary user is able to use getty, login, bash, ps, kill and similar
    things when one of her processes got out of control.

Since the current overcommit mode 2 is not really useful, I did not give
this a new number.

The patch is just for playing, not to be applied by Linus.  But, Andrew, I
hope that you would be willing to put this in -mm so that people can
experiment.  Of course it only does something if one sets overcommit mode
to 2.

The past month I have pressured people asking for feedback, and now have
about a dozen reports, mostly positive, one very positive.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>