[PATCH RFC 08/10] xen/balloon: implement migratepage

[Wei Liu <wei.liu2@citrix.com>]

This patch replaces the xen_balloon_migratepage stub with actual
implementation.

It's implemented in two macro steps:
1. populate old page with machine page and remove it from balloon driver
2. give up machine page that backs new page and queue it to balloon
   driver

Old page is the page owned by balloon driver, new page is a page
isolated by core mm compaction thread.

The aforementioned steps swaps a ballooned out page with a usable page.
>From guest's point of view, it de-fragments physical address space.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
---
 drivers/xen/balloon.c |   94 +++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 92 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 815e1d5..112190e 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -727,13 +727,103 @@ static struct notifier_block balloon_cpu_notifier = {
 
 static const struct address_space_operations xen_balloon_aops;
 #ifdef CONFIG_BALLOON_COMPACTION
+/*
+ * xen_balloon_migratepage - perform the balloon page migration on behalf of
+ *                           a compation thread.     (called under page lock)
+ * @mapping: the page->mapping which will be assigned to the new migrated page
+ * @newpage: page that will replace the isolated page after migration finishes
+ * @page   : the isolated (old) page that is about to be migrated to newpage.
+ * @mode   : compaction mode -- not used for balloon page migration.
+ *
+ * After a ballooned page gets isolated by compaction procedures, this
+ * is the function that performs the page migration on behalf of a
+ * compaction thread. The page migration for Xen balloon is done in
+ * these two macro steps:
+ *
+ *  A. back @page with machine page
+ *  B. release machine that backs @newpage
+ *
+ * Logically the above steps should work in reversed order (first B
+ * then A). But if we fail in A (in BA order) due to memory pressure
+ * in Xen we might not get back @newpage easily. With current order,
+ * we can safely return -EAGAIN if step A fails (either due to memory
+ * cap for guest or out of memory in hypervisor).
+ *
+ * This function preforms the balloon page migration task.
+ * Called through balloon_mapping->a_ops->migratepage
+ */
 static int xen_balloon_migratepage(struct address_space *mapping,
 				   struct page *newpage, struct page *page,
 				   enum migrate_mode mode)
 {
-	return -EAGAIN;
-}
+	struct xen_balloon *xb;
+	struct balloon_dev_info *info = balloon_page_device(page);
+	unsigned long flags;
+	unsigned long pfn;
+	xen_pfn_t frame;
+	int rc;
+
+	BUG_ON(!info);
+	BUG_ON(info->balloon_device != &xen_balloon);
+
+	xb = info->balloon_device;
+
+	/* Avoid contention if we're increasing / decreasing
+	 * reservation.
+	 */
+	if (!mutex_trylock(&xb->balloon_mutex))
+		return -EAGAIN;
+
+	kmap_flush_unused();
+
+	/*
+	 * Step A:
+	 * Back page with machine page
+	 */
+	frame = page_to_pfn(page);
+
+	rc = __memory_op_hypercall(XENMEM_populate_physmap, &frame, 1);
+	if (rc != 1) {
+		rc = -EAGAIN;
+		goto out;
+	}
+	/*
+	 * It's safe to delete page->lru here because this page is at
+	 * an isolated migration list, and this step is expected to happen here
+	 */
+	balloon_page_delete(page);
+
+	if (!xen_feature(XENFEAT_auto_translated_physmap))
+		__link_back_to_pagetable(page, frame,
+					 mfn_pte(frame, PAGE_KERNEL));
+
+	/*
+	 * Step B:
+	 * Give up newpage's backing machine page and add it to list
+	 */
+	pfn = page_to_pfn(newpage);
+	frame = pfn_to_mfn(pfn);
+	scrub_page(newpage);
 
+	if (!xen_feature(XENFEAT_auto_translated_physmap))
+		__replace_mapping_with_scratch_page(newpage);
+
+	rc = __memory_op_hypercall(XENMEM_decrease_reservation, &frame, 1);
+	BUG_ON(rc != 1);
+
+	spin_lock_irqsave(&info->pages_lock, flags);
+	balloon_page_insert(newpage, mapping, &info->pages);
+	info->isolated_pages--;
+	spin_unlock_irqrestore(&info->pages_lock, flags);
+
+	rc = MIGRATEPAGE_BALLOON_SUCCESS;
+
+	flush_tlb_all();
+out:
+	mutex_unlock(&xb->balloon_mutex);
+
+	return rc;
+}
 static const struct address_space_operations xen_balloon_aops = {
 	.migratepage = xen_balloon_migratepage,
 };
-- 
1.7.10.4