mm/balloon_compaction: remove dependency on page lock

Let's stop using the page lock in balloon code and instead use only the
balloon_device_lock.

As soon as we set the PG_movable_ops flag, we might now get isolation
callbacks for that page as we are no longer holding the page lock.  In
there, we'll simply synchronize using the balloon_device_lock.

So in balloon_page_isolate() lookup the balloon_dev_info through
page->private under balloon_device_lock.

It's crucial that we update page->private under the balloon_device_lock,
so the isolation callback can properly deal with concurrent deflation.

Consequently, make sure that balloon_page_finalize() is called under
balloon_device_lock as we remove a page from the list and clear
page->private.  balloon_page_insert() is already called with the
balloon_device_lock held.

Note that the core will still lock the pages, for example in
isolate_movable_ops_page().  The lock is there still relevant for handling
the PageMovableOpsIsolated flag, but that can be later changed to use an
atomic test-and-set instead, or moved into the movable_ops backends.

Link: https://lkml.kernel.org/r/20260119230133.3551867-10-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, 13 insertions, 12 deletions

diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index 9a8568fcd477..ad594af6ed10 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -12,25 +12,27 @@
  * is derived from the page type (PageOffline()) combined with the
  * PG_movable_ops flag (PageMovableOps()).
  *
+ * Once the page type and the PG_movable_ops are set, migration code
+ * can initiate page isolation by invoking the
+ * movable_operations()->isolate_page() callback
+ *
+ * As long as page->private is set, the page is either on the balloon list
+ * or isolated for migration. If page->private is not set, the page is
+ * either still getting inflated, or was deflated to be freed by the balloon
+ * driver soon. Isolation is impossible in both cases.
+ *
  * 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 simple rules:
  *
- *   i. Setting the PG_movable_ops flag and page->private with the following
- *	lock order
- *	    +-page_lock(page);
- *	      +--spin_lock_irq(&balloon_pages_lock);
+ *   i. Inflation/deflation must set/clear page->private under the
+ *      balloon_pages_lock
  *
  *  ii. isolation or dequeueing procedure must remove the page from balloon
  *      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
- * set of exposed rules are satisfied while we are dealing with balloon pages
- * compaction / migration.
- *
  * Copyright (C) 2012, Red Hat, Inc.  Rafael Aquini <aquini@redhat.com>
  */
 #ifndef _LINUX_BALLOON_COMPACTION_H
@@ -93,8 +95,7 @@ static inline struct balloon_dev_info *balloon_page_device(struct page *page)
  * @balloon : pointer to balloon device
  * @page    : page to be assigned as a 'balloon page'
  *
- * Caller must ensure the page is locked and the spin_lock protecting balloon
- * pages list is held before inserting a page into the balloon device.
+ * Caller must ensure the balloon_pages_lock is held.
  */
 static inline void balloon_page_insert(struct balloon_dev_info *balloon,
 				       struct page *page)
@@ -119,7 +120,7 @@ static inline gfp_t balloon_mapping_gfp_mask(void)
  *			   balloon list for release to the page allocator
  * @page: page to be released to the page allocator
  *
- * Caller must ensure that the page is locked.
+ * Caller must ensure the balloon_pages_lock is held.
  */
 static inline void balloon_page_finalize(struct page *page)
 {