[PATCH v10 3/3] mm: Maintain randomization of page free lists

[Dan Williams]

When freeing a page with an order >= shuffle_page_order randomly select
the front or back of the list for insertion.

While the mm tries to defragment physical pages into huge pages this can
tend to make the page allocator more predictable over time. Inject the
front-back randomness to preserve the initial randomness established by
shuffle_free_memory() when the kernel was booted.

The overhead of this manipulation is constrained by only being applied
for MAX_ORDER sized pages by default.

Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 include/linux/mmzone.h |   12 ++++++++++++
 mm/page_alloc.c        |   11 +++++++++--
 mm/shuffle.c           |   23 +++++++++++++++++++++++
 mm/shuffle.h           |   12 ++++++++++++
 4 files changed, 56 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 2274e43933ae..a3cb9a21196d 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -116,6 +116,18 @@ static inline void add_to_free_area_tail(struct page *page, struct free_area *ar
 	area->nr_free++;
 }
 
+#ifdef CONFIG_SHUFFLE_PAGE_ALLOCATOR
+/* Used to preserve page allocation order entropy */
+void add_to_free_area_random(struct page *page, struct free_area *area,
+		int migratetype);
+#else
+static inline void add_to_free_area_random(struct page *page,
+		struct free_area *area, int migratetype)
+{
+	add_to_free_area(page, area, migratetype);
+}
+#endif
+
 /* Used for pages which are on another list */
 static inline void move_to_free_area(struct page *page, struct free_area *area,
 			     int migratetype)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3fd0df403766..2a0969e3b0eb 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -43,6 +43,7 @@
 #include <linux/mempolicy.h>
 #include <linux/memremap.h>
 #include <linux/stop_machine.h>
+#include <linux/random.h>
 #include <linux/sort.h>
 #include <linux/pfn.h>
 #include <linux/backing-dev.h>
@@ -889,7 +890,8 @@ static inline void __free_one_page(struct page *page,
 	 * so it's less likely to be used soon and more likely to be merged
 	 * as a higher order page
 	 */
-	if ((order < MAX_ORDER-2) && pfn_valid_within(buddy_pfn)) {
+	if ((order < MAX_ORDER-2) && pfn_valid_within(buddy_pfn)
+			&& !is_shuffle_order(order)) {
 		struct page *higher_page, *higher_buddy;
 		combined_pfn = buddy_pfn & pfn;
 		higher_page = page + (combined_pfn - pfn);
@@ -903,7 +905,12 @@ static inline void __free_one_page(struct page *page,
 		}
 	}
 
-	add_to_free_area(page, &zone->free_area[order], migratetype);
+	if (is_shuffle_order(order))
+		add_to_free_area_random(page, &zone->free_area[order],
+				migratetype);
+	else
+		add_to_free_area(page, &zone->free_area[order], migratetype);
+
 }
 
 /*
diff --git a/mm/shuffle.c b/mm/shuffle.c
index 8badf4f0a852..19bbf3e37fb6 100644
--- a/mm/shuffle.c
+++ b/mm/shuffle.c
@@ -168,3 +168,26 @@ void __meminit __shuffle_free_memory(pg_data_t *pgdat)
 	for (z = pgdat->node_zones; z < pgdat->node_zones + MAX_NR_ZONES; z++)
 		shuffle_zone(z);
 }
+
+void add_to_free_area_random(struct page *page, struct free_area *area,
+		int migratetype)
+{
+	static u64 rand;
+	static u8 rand_bits;
+
+	/*
+	 * The lack of locking is deliberate. If 2 threads race to
+	 * update the rand state it just adds to the entropy.
+	 */
+	if (rand_bits == 0) {
+		rand_bits = 64;
+		rand = get_random_u64();
+	}
+
+	if (rand & 1)
+		add_to_free_area(page, area, migratetype);
+	else
+		add_to_free_area_tail(page, area, migratetype);
+	rand_bits--;
+	rand >>= 1;
+}
diff --git a/mm/shuffle.h b/mm/shuffle.h
index 644c8ee97b9e..fc1e327ae22d 100644
--- a/mm/shuffle.h
+++ b/mm/shuffle.h
@@ -36,6 +36,13 @@ static inline void shuffle_zone(struct zone *z)
 		return;
 	__shuffle_zone(z);
 }
+
+static inline bool is_shuffle_order(int order)
+{
+	if (!static_branch_unlikely(&page_alloc_shuffle_key))
+                return false;
+	return order >= SHUFFLE_ORDER;
+}
 #else
 static inline void shuffle_free_memory(pg_data_t *pgdat)
 {
@@ -48,5 +55,10 @@ static inline void shuffle_zone(struct zone *z)
 static inline void page_alloc_shuffle(enum mm_shuffle_ctl ctl)
 {
 }
+
+static inline bool is_shuffle_order(int order)
+{
+	return false;
+}
 #endif
 #endif /* _MM_SHUFFLE_H */