linux-kernel - [PATCH RFC 28/29] mm/balloon_compaction: "movable

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Message-ID: <20250618174014.1168640-29-david@redhat.com>
Date: Wed, 18 Jun 2025 19:40:11 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
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Subject: [PATCH RFC 28/29] mm/balloon_compaction: "movable_ops" doc updates

Let's bring the docs up-to-date. Setting PG_movable_ops + page->private
very likely still requires to be performed under documented locks:
it's complicated.

We will rework this in the future, as we will try avoiding using the
page lock.

Signed-off-by: David Hildenbrand <david@...hat.com>
---
 include/linux/balloon_compaction.h | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index b222b0737c466..2fecfead91d26 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -4,12 +4,13 @@
  *
  * Common interface definitions for making balloon pages movable by compaction.
  *
- * Balloon page migration makes use of the general non-lru movable page
+ * Balloon page migration makes use of the general "movable_ops page migration"
  * 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.
+ * That these pages have movable_ops, and which movable_ops apply,
+ * is derived from the page type (PageOffline()) combined with the
+ * PG_movable_ops flag (PageMovableOps()).
  *
  * 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
@@ -17,12 +18,10 @@
  * and safely perform balloon's page compaction and migration we must, always,
  * 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
- *      locking scheme (lru_lock, balloon_lock):
+ *   i. Setting the PG_movable_ops flag and page->private with the following
+ *	lock order
  *	    +-page_lock(page);
  *	      +--spin_lock_irq(&b_dev_info->pages_lock);
- *	            ... page->mapping updates here ...
  *
  *  ii. isolation or dequeueing procedure must remove the page from balloon
  *      device page list under b_dev_info->pages_lock.
-- 
2.49.0