memfd_restricted() itself is implemented as a shim layer on top of real
memory file systems (currently tmpfs). Pages in restrictedmem are marked
as unmovable and unevictable, this is required for current confidential
usage. But in future this might be changed.

I didn't dig full histroy, but I interpret this as we don't support page
migration and swapping for restricted memfd for now.  IMHO "page marked as
unmovable" can be confused with PageMovable(), which is a different thing from
this series.  It's better to just say something like "those pages cannot be
migrated and swapped".

[...]

+
+	/*
+	 * These pages are currently unmovable so don't place them into movable
+	 * pageblocks (e.g. CMA and ZONE_MOVABLE).
+	 */
+	mapping = memfd->f_mapping;
+	mapping_set_unevictable(mapping);
+	mapping_set_gfp_mask(mapping,
+			     mapping_gfp_mask(mapping) & ~__GFP_MOVABLE);

But, IIUC removing __GFP_MOVABLE flag here only makes page allocation from non-
movable zones, but doesn't necessarily prevent page from being migrated.  My
first glance is you need to implement either a_ops->migrate_folio() or just
get_page() after faulting in the page to prevent.

So I think the comment also needs improvement -- IMHO we can just call out
currently those pages cannot be migrated and swapped, which is clearer (and the
latter justifies mapping_set_unevictable() clearly).