Do not disable interrupts when reading min_free_kbytes

The sysctl handler for min_free_kbytes calls setup_per_zone_pages_min() on
read or write.  This function iterates through every zone and calls
spin_lock_irqsave() on the zone LRU lock.  When reading min_free_kbytes,
this is a total waste of time that disables interrupts on the local
processor.  It might even be noticable machines with large numbers of zones
if a process started constantly reading min_free_kbytes.

This patch only calls setup_per_zone_pages_min() only on write. Tested on
an x86 laptop and it did the right thing.
Signed-off-by: NMel Gorman <mel@csn.ul.ie>
Acked-by: NChristoph Lameter <clameter@engr.sgi.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -3199,7 +3199,8 @@ int min_free_kbytes_sysctl_handler(ctl_table *table, int write,
 	struct file *file, void __user *buffer, size_t *length, loff_t *ppos)
 {
 	proc_dointvec(table, write, file, buffer, length, ppos);
-	setup_per_zone_pages_min();
+	if (write)
+		setup_per_zone_pages_min();
 	return 0;
 }