From 5b0087cbb80eceadc1d4b05b587868abf0d9cf71 Mon Sep 17 00:00:00 2001
From: Hauke Mehrtens <hauke@hauke-m.de>
Date: Sun, 8 Jun 2014 22:52:39 +0200
Subject: [PATCH] backports: add split_page()

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
---
 backport/compat/backport-3.10.c | 46 +++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/backport/compat/backport-3.10.c b/backport/compat/backport-3.10.c
index 9bcb8059506a..e75e52ed8164 100644
--- a/backport/compat/backport-3.10.c
+++ b/backport/compat/backport-3.10.c
@@ -17,6 +17,7 @@
 #include <linux/pci.h>
 #include <linux/pci_regs.h>
 #include <linux/of.h>
+#include <linux/mm.h>
 
 #if (LINUX_VERSION_CODE >= KERNEL_VERSION(3,6,0))
 #include <linux/init.h>
@@ -232,3 +233,48 @@ int of_property_read_u32_index(const struct device_node *np,
 }
 EXPORT_SYMBOL_GPL(of_property_read_u32_index);
 #endif /* CONFIG_OF */
+
+static inline void set_page_count(struct page *page, int v)
+{
+	atomic_set(&page->_count, v);
+}
+
+/*
+ * Turn a non-refcounted page (->_count == 0) into refcounted with
+ * a count of one.
+ */
+static inline void set_page_refcounted(struct page *page)
+{
+	VM_BUG_ON(PageTail(page));
+	VM_BUG_ON(atomic_read(&page->_count));
+	set_page_count(page, 1);
+}
+
+/*
+ * split_page takes a non-compound higher-order page, and splits it into
+ * n (1<<order) sub-pages: page[0..n]
+ * Each sub-page must be freed individually.
+ *
+ * Note: this is probably too low level an operation for use in drivers.
+ * Please consult with lkml before using this in your driver.
+ */
+void split_page(struct page *page, unsigned int order)
+{
+	int i;
+
+	VM_BUG_ON(PageCompound(page));
+	VM_BUG_ON(!page_count(page));
+
+#ifdef CONFIG_KMEMCHECK
+	/*
+	 * Split shadow pages too, because free(page[0]) would
+	 * otherwise free the whole shadow.
+	 */
+	if (kmemcheck_page_is_tracked(page))
+		split_page(virt_to_page(page[0].shadow), order);
+#endif
+
+	for (i = 1; i < (1 << order); i++)
+		set_page_refcounted(page + i);
+}
+EXPORT_SYMBOL_GPL(split_page);
-- 
2.30.2