mm/compaction: optimize >0 order folio compaction with free page split.

During migration in a memory compaction, free pages are placed in an array
of page lists based on their order.  But the desired free page order
(i.e., the order of a source page) might not be always present, thus
leading to migration failures and premature compaction termination.  Split
a high order free pages when source migration page has a lower order to
increase migration successful rate.

Note: merging free pages when a migration fails and a lower order free
page is returned via compaction_free() is possible, but there is too much
work.  Since the free pages are not buddy pages, it is hard to identify
these free pages using existing PFN-based page merging algorithm.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Zi Yan <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Tested-by: Baolin Wang <[email protected]>
Tested-by: Yu Zhao <[email protected]>
Cc: Adam Manzanares <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Kemeng Shi <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Vishal Moola (Oracle) <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Yin Fengwei <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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