[PATCH v5, part3 11/15] mm: use a dedicated lock to protect totalram_pages and zone->managed_pages

[Jiang Liu]

Currently lock_memory_hotplug()/unlock_memory_hotplug() are used to
protect totalram_pages and zone->managed_pages. Other than the memory
hotplug driver, totalram_pages and zone->managed_pages may also be
modified at runtime by other drivers, such as Xen balloon,
virtio_balloon etc. For those cases, memory hotplug lock is a little
too heavy, so introduce a dedicated lock to protect totalram_pages
and zone->managed_pages.

Now we have a simplified locking rules totalram_pages and
zone->managed_pages as:
1) no locking for read accesses because they are unsigned long.
2) no locking for write accesses at boot time in single-threaded context.
3) serialize write accesses at runtime by acquiring the dedicated
   managed_page_count_lock.

Also adjust zone->managed_pages when freeing reserved pages into the
buddy system, to keep totalram_pages and zone->managed_pages in
consistence.

Signed-off-by: Jiang Liu <jiang.liu@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Michel Lespinasse <walken@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx (open list:MEMORY MANAGEMENT)
Cc: linux-kernel@xxxxxxxxxxxxxxx (open list)
---
 include/linux/mm.h     |    6 ++----
 include/linux/mmzone.h |   14 ++++++++++----
 mm/page_alloc.c        |   12 ++++++++++++
 3 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9f7006f..86014c9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1303,6 +1303,7 @@ extern void free_initmem(void);
  */
 extern unsigned long free_reserved_area(unsigned long start, unsigned long end,
 					int poison, char *s);
+
 #ifdef	CONFIG_HIGHMEM
 /*
  * Free a highmem page into the buddy system, adjusting totalhigh_pages
@@ -1311,10 +1312,7 @@ extern unsigned long free_reserved_area(unsigned long start, unsigned long end,
 extern void free_highmem_page(struct page *page);
 #endif
 
-static inline void adjust_managed_page_count(struct page *page, long count)
-{
-	totalram_pages += count;
-}
+extern void adjust_managed_page_count(struct page *page, long count);
 
 /* Free the reserved page into the buddy system, so it gets managed. */
 static inline void __free_reserved_page(struct page *page)
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 8c9f859..14ca1a9 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -474,10 +474,16 @@ struct zone {
 	 * frequently read in proximity to zone->lock.  It's good to
 	 * give them a chance of being in the same cacheline.
 	 *
-	 * Write access to present_pages and managed_pages at runtime should
-	 * be protected by lock_memory_hotplug()/unlock_memory_hotplug().
-	 * Any reader who can't tolerant drift of present_pages and
-	 * managed_pages should hold memory hotplug lock to get a stable value.
+	 * Write access to present_pages at runtime should be protected by
+	 * lock_memory_hotplug()/unlock_memory_hotplug().  Any reader who can't
+	 * tolerant drift of present_pages should hold memory hotplug lock to
+	 * get a stable value.
+	 *
+	 * Read access to managed_pages should be safe because it's unsigned
+	 * long. Write access to zone->managed_pages and totalram_pages are
+	 * protected by managed_page_count_lock at runtime. Idealy only
+	 * adjust_managed_page_count() should be used instead of directly
+	 * touching zone->managed_pages and totalram_pages.
 	 */
 	unsigned long		spanned_pages;
 	unsigned long		present_pages;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c5c0988..9d8209a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -100,6 +100,9 @@ nodemask_t node_states[NR_NODE_STATES] __read_mostly = {
 };
 EXPORT_SYMBOL(node_states);
 
+/* Protect totalram_pages and zone->managed_pages */
+static DEFINE_SPINLOCK(managed_page_count_lock);
+
 unsigned long totalram_pages __read_mostly;
 unsigned long totalreserve_pages __read_mostly;
 /*
@@ -5186,6 +5189,15 @@ early_param("movablecore", cmdline_parse_movablecore);
 
 #endif /* CONFIG_HAVE_MEMBLOCK_NODE_MAP */
 
+void adjust_managed_page_count(struct page *page, long count)
+{
+	spin_lock(&managed_page_count_lock);
+	page_zone(page)->managed_pages += count;
+	totalram_pages += count;
+	spin_unlock(&managed_page_count_lock);
+}
+EXPORT_SYMBOL(adjust_managed_page_count);
+
 unsigned long free_reserved_area(unsigned long start, unsigned long end,
 				 int poison, char *s)
 {
-- 
1.7.9.5

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