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Subject[PATCH RFC 1/6] mm: balloon: update comment about isolation/migration/compaction
Commit b1123ea6d3b3 ("mm: balloon: use general non-lru movable page
feature") reworked balloon handling to make use of the general
non-lru movable page feature. The big comment block in
balloon_compaction.h contains quite some outdated information. Let's fix

Cc: Andrew Morton <>
Cc: Matthew Wilcox <>
Cc: Michal Hocko <>
Cc: "Michael S. Tsirkin" <>
Signed-off-by: David Hildenbrand <>
include/linux/balloon_compaction.h | 26 +++++++++-----------------
1 file changed, 9 insertions(+), 17 deletions(-)

diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index 53051f3d8f25..cbe50da5a59d 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -4,15 +4,18 @@
* Common interface definitions for making balloon pages movable by compaction.
- * Despite being perfectly possible to perform ballooned pages migration, they
- * make a special corner case to compaction scans because balloon pages are not
- * enlisted at any LRU list like the other pages we do compact / migrate.
+ * Balloon page migration makes use of the general non-lru movable page
+ * feature.
+ *
+ * page->private is used to reference the responsible balloon device.
+ * page->mapping is used in context of non-lru page migration to reference
+ * the address space operations for page isolation/migration/compaction.
* As the page isolation scanning step a compaction thread does is a lockless
* procedure (from a page standpoint), it might bring some racy situations while
* performing balloon page compaction. In order to sort out these racy scenarios
* and safely perform balloon's page compaction and migration we must, always,
- * ensure following these three simple rules:
+ * ensure following these simple rules:
* i. when updating a balloon's page ->mapping element, strictly do it under
* the following lock order, independently of the far superior
@@ -21,19 +24,8 @@
* +--spin_lock_irq(&b_dev_info->pages_lock);
* ... page->mapping updates here ...
- * ii. before isolating or dequeueing a balloon page from the balloon device
- * pages list, the page reference counter must be raised by one and the
- * extra refcount must be dropped when the page is enqueued back into
- * the balloon device page list, thus a balloon page keeps its reference
- * counter raised only while it is under our special handling;
- *
- * iii. after the lockless scan step have selected a potential balloon page for
- * isolation, re-test the PageBalloon mark and the PagePrivate flag
- * under the proper page lock, to ensure isolating a valid balloon page
- * (not yet isolated, nor under release procedure)
- *
- * iv. isolation or dequeueing procedure must clear PagePrivate flag under
- * page lock together with removing page from balloon device page list.
+ * ii. isolation or dequeueing procedure must remove the page from balloon
+ * device page list under b_dev_info->pages_lock.
* The functions provided by this interface are placed to help on coping with
* the aforementioned balloon page corner case, as well as to ensure the simple
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