[PATCH] UML: Hotplug memory, take 2

[PATCH] UML: Hotplug memory, take 2

Changes since first version
	added check for MADV_REMOVE support on the host
	fixed error return botch
	shrunk sprintf array by one character

This adds hotplug memory support to UML.  The mconsole syntax is
 	config mem=[+-]n[KMG]
In other words, add or subtract some number of kilobytes, megabytes, or
gigabytes.

Unplugged pages are allocated and then madvise(MADV_TRUNCATE), which is a
currently experimental madvise extension.  These pages are tracked so they
can be plugged back in later if the admin decides to give them back.  The
first page to be unplugged is used to keep track of about 4M of other
pages.  A list_head is the first thing on this page.  The rest is filled
with addresses of other unplugged pages.  This first page is not madvised,
obviously.

When this page is filled, the next page is used in a similar way and linked
onto a list with the first page.  Etc.  This whole process reverses when
pages are plugged back in.  When a tracking page no longer tracks any
unplugged pages, then it is next in line for plugging, which is done by
freeing pages back to the kernel.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>