[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>

1 files changed, 15 insertions, 0 deletions

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index e050d68963a1..f5fa3082fd6a 100644
--- a/include/linux/mmzone.h
+++ b/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) */