mm/balloon_compaction: use a device-independent balloon (list) lock

In order to remove the dependency on the page lock for balloon pages, we
need a lock that is independent of the page.

It's crucial that we can handle the scenario where balloon deflation
(clearing page->private) can race with page isolation (using page->private
to obtain the balloon_dev_info where the lock currently resides).

The current lock in balloon_dev_info is therefore not suitable.

Fortunately, we never really have more than a single balloon device per
VM, so we can just keep it simple and use a static lock to protect all
balloon devices.

Based on this change we will remove the dependency on the page lock next.

Link: https://lkml.kernel.org/r/20260119230133.3551867-9-david@kernel.org
Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Eugenio Pérez <eperezma@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Jerrin Shaji George <jerrin.shaji-george@broadcom.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: SeongJae Park <sj@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 2 insertions, 4 deletions

diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index 3109d3c43d30..9a8568fcd477 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -21,10 +21,10 @@
  *   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);
+ *	      +--spin_lock_irq(&balloon_pages_lock);
  *
  *  ii. isolation or dequeueing procedure must remove the page from balloon
- *      device page list under b_dev_info->pages_lock.
+ *      device page list under balloon_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
@@ -52,7 +52,6 @@
  */
 struct balloon_dev_info {
 	unsigned long isolated_pages;	/* # of isolated pages for migration */
-	spinlock_t pages_lock;		/* Protection to pages list */
 	struct list_head pages;		/* Pages enqueued & handled to Host */
 	int (*migratepage)(struct balloon_dev_info *, struct page *newpage,
 			struct page *page, enum migrate_mode mode);
@@ -71,7 +70,6 @@ extern size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info,
 static inline void balloon_devinfo_init(struct balloon_dev_info *balloon)
 {
 	balloon->isolated_pages = 0;
-	spin_lock_init(&balloon->pages_lock);
 	INIT_LIST_HEAD(&balloon->pages);
 	balloon->migratepage = NULL;
 	balloon->adjust_managed_page_count = false;