[PATCH] memory hotplug locking: zone span seqlock

See the "fixup bad_range()" patch for more information, but this actually
creates a the lock to protect things making assumptions about a zone's size
staying constant at runtime.

Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/mmzone.h

@@ -12,6 +12,7 @@
 #include <linux/threads.h>
 #include <linux/numa.h>
 #include <linux/init.h>
+#include <linux/seqlock.h>
 #include <asm/atomic.h>
 
 /* Free memory management - zoned buddy allocator.  */
@@ -137,6 +138,10 @@ struct zone {
 	 * free areas of different sizes
 	 */
 	spinlock_t		lock;
+#ifdef CONFIG_MEMORY_HOTPLUG
+	/* see spanned/present_pages for more description */
+	seqlock_t		span_seqlock;
+#endif
 	struct free_area	free_area[MAX_ORDER];
 
 
@@ -220,6 +225,16 @@ struct zone {
 	/* zone_start_pfn == zone_start_paddr >> PAGE_SHIFT */
 	unsigned long		zone_start_pfn;
 
+	/*
+	 * zone_start_pfn, spanned_pages and present_pages are all
+	 * protected by span_seqlock.  It is a seqlock because it has
+	 * to be read outside of zone->lock, and it is done in the main
+	 * allocator path.  But, it is written quite infrequently.
+	 *
+	 * The lock is declared along with zone->lock because it is
+	 * frequently read in proximity to zone->lock.  It's good to
+	 * give them a chance of being in the same cacheline.
+	 */
 	unsigned long		spanned_pages;	/* total size, including holes */
 	unsigned long		present_pages;	/* amount of memory (excluding holes) */