mm: rename CONFIG_BALLOON_COMPACTION to CONFIG_BALLOON_MIGRATION

While compaction depends on migration, the other direction is not the
case.  So let's make it clearer that this is all about migration of
balloon pages.

Adjust all comments/docs in the core to talk about "migration" instead of
"compaction".

While at it add some "/* CONFIG_BALLOON_MIGRATION */".

Link: https://lkml.kernel.org/r/20260119230133.3551867-23-david@kernel.org
Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
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: 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>

drivers/virtio/virtio_balloon.c

@@ -802,7 +802,7 @@ static void report_free_page_func(struct work_struct *work)
 	}
 }
 
-#ifdef CONFIG_BALLOON_COMPACTION
+#ifdef CONFIG_BALLOON_MIGRATION
 /*
  * virtballoon_migratepage - perform the balloon page migration on behalf of
  *			     a compaction thread.     (called under page lock)
@@ -851,7 +851,7 @@ static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
 	mutex_unlock(&vb->balloon_lock);
 	return 0;
 }
-#endif /* CONFIG_BALLOON_COMPACTION */
+#endif /* CONFIG_BALLOON_MIGRATION */
 
 static unsigned long shrink_free_pages(struct virtio_balloon *vb,
 				       unsigned long pages_to_free)
@@ -948,7 +948,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
 
 	if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
 		vb->vb_dev_info.adjust_managed_page_count = true;
-#ifdef CONFIG_BALLOON_COMPACTION
+#ifdef CONFIG_BALLOON_MIGRATION
 	vb->vb_dev_info.migratepage = virtballoon_migratepage;
 #endif
 	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {