vm.txt: document min_free_pages as critical for correctness

min_free_pages is critical for correctness, document it as such.
Signed-off-by: NPavel Machek <pavel@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/sysctl/vm.txt

@@ -112,6 +112,12 @@ of kilobytes free.  The VM uses this number to compute a pages_min
 value for each lowmem zone in the system.  Each lowmem zone gets 
 a number of reserved free pages based proportionally on its size.
 
+Some minimal ammount of memory is needed to satisfy PF_MEMALLOC
+allocations; if you set this to lower than 1024KB, your system will
+become subtly broken, and prone to deadlock under high loads.
+
+Setting this too high will OOM your machine instantly.
+
 ==============================================================
 
 percpu_pagelist_fraction