mm/migrate: remove MIGRATEPAGE_UNMAP

mm/migrate: remove MIGRATEPAGE_UNMAP

migrate_folio_unmap() is the only user of MIGRATEPAGE_UNMAP.  We want to
remove MIGRATEPAGE_* completely.

It's rather weird to have a generic MIGRATEPAGE_UNMAP, documented to be
returned from address-space callbacks, when it's only used for an internal
helper.

Let's start by having only a single "success" return value for
migrate_folio_unmap() -- 0 -- by moving the "folio was already freed"
check into the single caller.

There is a remaining comment for PG_isolated, which we renamed to
PG_movable_ops_isolated recently and forgot to update.

While we might still run into that case with zsmalloc, it's something we
want to get rid of soon.  So let's just focus that optimization on real
folios only for now by excluding movable_ops pages.  Note that concurrent
freeing can happen at any time and this "already freed" check is not
relevant for correctness.

[david@redhat.com: no need to pass "reason" to migrate_folio_unmap(), per Lance]
Link: https://lkml.kernel.org/r/3bb725f8-28d7-4aa2-b75f-af40d5cab280@redhat.com
Link: https://lkml.kernel.org/r/20250811143949.1117439-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Chris Mason <clm@fb.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Dave Kleikamp <shaggy@kernel.org>
Cc: David Sterba <dsterba@suse.com>
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 Wang <jasowang@redhat.com>
Cc: Jerrin Shaji George <jerrin.shaji-george@broadcom.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>