Bug#695182: [PATCH] Subtract min_free_kbytes from dirtyable memory

[paul.szabo@sydney.edu.au]

When calculating amount of dirtyable memory, min_free_kbytes should be
subtracted because it is not intended for dirty pages.

Using an "extern int" because that is the only interface to some such
sysctl values.

(This patch does not solve the PAE OOM issue.)

Paul Szabo   psz@maths.usyd.edu.au   http://www.maths.usyd.edu.au/u/psz/
School of Mathematics and Statistics   University of Sydney    Australia

Reported-by: Paul Szabo <psz@maths.usyd.edu.au>
Reference: http://bugs.debian.org/695182
Signed-off-by: Paul Szabo <psz@maths.usyd.edu.au>

--- mm/page-writeback.c.old	2012-12-06 22:20:40.000000000 +1100
+++ mm/page-writeback.c	2013-01-21 13:57:05.000000000 +1100
@@ -343,12 +343,16 @@
 unsigned long determine_dirtyable_memory(void)
 {
 	unsigned long x;
+	extern int min_free_kbytes;
 
 	x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();
 
 	if (!vm_highmem_is_dirtyable)
 		x -= highmem_dirtyable_memory(x);
 
+	/* Subtract min_free_kbytes */
+	x -= min(x, min_free_kbytes >> (PAGE_SHIFT - 10));
+
 	return x + 1;	/* Ensure that we never return 0 */
 }