[PATCH v2] mm/balloon_compaction: avoid duplicate page removal

[Wei Wang <wei.w.wang@xxxxxxxxx>]

Fixes: 418a3ab1e778 (mm/balloon_compaction: List interfaces)

A #GP is reported in the guest when requesting balloon inflation via
virtio-balloon. The reason is that the virtio-balloon driver has
removed the page from its internal page list (via balloon_page_pop),
but balloon_page_enqueue_one also calls "list_del"  to do the removal.
This is necessary when it's used from balloon_page_enqueue_list, but
not from balloon_page_enqueue_one.

So remove the list_del balloon_page_enqueue_one, and update some
comments as a reminder.

Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
---
ChangeLong:
v1->v2: updated some comments

 mm/balloon_compaction.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index 83a7b61..8639bfc 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c
@@ -21,7 +21,6 @@ static void balloon_page_enqueue_one(struct balloon_dev_info *b_dev_info,
 	 * memory corruption is possible and we should stop execution.
 	 */
 	BUG_ON(!trylock_page(page));
-	list_del(&page->lru);
 	balloon_page_insert(b_dev_info, page);
 	unlock_page(page);
 	__count_vm_event(BALLOON_INFLATE);
@@ -33,7 +32,7 @@ static void balloon_page_enqueue_one(struct balloon_dev_info *b_dev_info,
  * @b_dev_info: balloon device descriptor where we will insert a new page to
  * @pages: pages to enqueue - allocated using balloon_page_alloc.
  *
- * Driver must call it to properly enqueue a balloon pages before definitively
+ * Driver must call it to properly enqueue balloon pages before definitively
  * removing it from the guest system.
  *
  * Return: number of pages that were enqueued.
@@ -47,6 +46,7 @@ size_t balloon_page_list_enqueue(struct balloon_dev_info *b_dev_info,
 
 	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
 	list_for_each_entry_safe(page, tmp, pages, lru) {
+		list_del(&page->lru);
 		balloon_page_enqueue_one(b_dev_info, page);
 		n_pages++;
 	}
@@ -128,13 +128,19 @@ struct page *balloon_page_alloc(void)
 EXPORT_SYMBOL_GPL(balloon_page_alloc);
 
 /*
- * balloon_page_enqueue - allocates a new page and inserts it into the balloon
- *			  page list.
+ * balloon_page_enqueue - inserts a new page into the balloon page list.
+ *
  * @b_dev_info: balloon device descriptor where we will insert a new page to
  * @page: new page to enqueue - allocated using balloon_page_alloc.
  *
  * Driver must call it to properly enqueue a new allocated balloon page
  * before definitively removing it from the guest system.
+ *
+ * Drivers must not call balloon_page_enqueue on pages that have been
+ * pushed to a list with balloon_page_push before removing them with
+ * balloon_page_pop. To all pages on a list, use balloon_page_list_enqueue
+ * instead.
+ *
  * This function returns the page address for the recently enqueued page or
  * NULL in the case we fail to allocate a new page this turn.
  */
-- 
2.7.4