With Rik van Riel <riel@redhat.com>

Simply running "dd if=/dev/zero of=/dev/hd<one you can miss>" can cause
excessive amounts of dirty lowmem pagecache, due to the blockdev's
non-highmem page allocation strategy.

This patch effectively lowers the dirty limit for mappings which cannot be
cached in highmem, counting the dirty limit as a percentage of lowmem
instead.  This should prevent heavy block device writers from pushing the
VM over the edge and triggering OOM kills.
Signed-off-by: Rik van Riel <riel@redhat.com>
Acked-by: Andrea Arcangeli <andrea@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>