mm/page_alloc: move pages to tail in move_to_free_list()

Whenever we move pages between freelists via move_to_free_list()/
move_freepages_block(), we don't actually touch the pages:
1. Page isolation doesn't actually touch the pages, it simply isolates
   pageblocks and moves all free pages to the MIGRATE_ISOLATE freelist.
   When undoing isolation, we move the pages back to the target list.
2. Page stealing (steal_suitable_fallback()) moves free pages directly
   between lists without touching them.
3. reserve_highatomic_pageblock()/unreserve_highatomic_pageblock() moves
   free pages directly between freelists without touching them.

We already place pages to the tail of the freelists when undoing isolation
via __putback_isolated_page(), let's do it in any case (e.g., if order <=
pageblock_order) and document the behavior. To simplify, let's move the
pages to the tail for all move_to_free_list()/move_freepages_block() users.

In 2., the target list is empty, so there should be no change.  In 3., we
might observe a change, however, highatomic is more concerned about
allocations succeeding than cache hotness - if we ever realize this change
degrades a workload, we can special-case this instance and add a proper
comment.

This change results in all pages getting onlined via online_pages() to be
placed to the tail of the freelist.

Signed-off-by: David Hildenbrand <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Reviewed-by: Wei Yang <[email protected]>
Acked-by: Pankaj Gupta <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Alexander Duyck <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Scott Cheloha <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Haiyang Zhang <[email protected]>
Cc: "K. Y. Srinivasan" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: Wei Liu <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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