docs/mm: convert from "Non-LRU page migration" to "movable_ops page migration"

Let's bring the docs up-to-date.

Link: https://lkml.kernel.org/r/20250704102524.326966-28-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Eugenio Pé rez <eperezma@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Jerrin Shaji George <jerrin.shaji-george@broadcom.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/mm/page_migration.rst

@@ -146,18 +146,33 @@ Steps:
 18. The new page is moved to the LRU and can be scanned by the swapper,
     etc. again.
 
-Non-LRU page migration
-======================
+movable_ops page migration
+==========================
 
-Although migration originally aimed for reducing the latency of memory
-accesses for NUMA, compaction also uses migration to create high-order
-pages.  For compaction purposes, it is also useful to be able to move
-non-LRU pages, such as zsmalloc and virtio-balloon pages.
+Selected typed, non-folio pages (e.g., pages inflated in a memory balloon,
+zsmalloc pages) can be migrated using the movable_ops migration framework.
 
-If a driver wants to make its pages movable, it should define a struct
-movable_operations.  It then needs to call __SetPageMovable() on each
-page that it may be able to move.  This uses the ``page->mapping`` field,
-so this field is not available for the driver to use for other purposes.
+The "struct movable_operations" provide callbacks specific to a page type
+for isolating, migrating and un-isolating (putback) these pages.
+
+Once a page is indicated as having movable_ops, that condition must not
+change until the page was freed back to the buddy. This includes not
+changing/clearing the page type and not changing/clearing the
+PG_movable_ops page flag.
+
+Arbitrary drivers cannot currently make use of this framework, as it
+requires:
+
+(a) a page type
+(b) indicating them as possibly having movable_ops in page_has_movable_ops()
+    based on the page type
+(c) returning the movable_ops from page_movable_ops() based on the page
+    type
+(d) not reusing the PG_movable_ops and PG_movable_ops_isolated page flags
+    for other purposes
+
+For example, balloon drivers can make use of this framework through the
+balloon-compaction infrastructure residing in the core kernel.
 
 Monitoring Migration
 =====================