mm: compaction: don't depend on HUGETLB_PAGE

Commit 5d68924 ("thp: select CONFIG_COMPACTION if TRANSPARENT_HUGEPAGE
enabled") causes this warning during the configuration process:

  warning: (TRANSPARENT_HUGEPAGE) selects COMPACTION which has unmet
  direct dependencies (EXPERIMENTAL && HUGETLB_PAGE && MMU)

COMPACTION doesn't depend on HUGETLB_PAGE, it doesn't depend on THP
either, it is also useful for regular alloc_pages(order > 0) including
the very kernel stack during fork (THREAD_ORDER = 1).  It's always
better to enable COMPACTION.

The warning should be an error because we would end up with MIGRATION
not selected, and COMPACTION wouldn't work without migration (despite it
seems to build with an inline migrate_pages returning -ENOSYS).

I'd also like to remove EXPERIMENTAL: compaction has been in the kernel
for some releases (for full safety the default remains disabled which I
think is enough).

Signed-off-by: Andrea Arcangeli <[email protected]>
Reported-by: Luca Tettamanti <[email protected]>
Tested-by: Luca Tettamanti <[email protected]>
Cc: Mel Gorman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/Kconfig

@@ -179,7 +179,7 @@ config SPLIT_PTLOCK_CPUS
 config COMPACTION
 	bool "Allow for memory compaction"
 	select MIGRATION
-	depends on EXPERIMENTAL && HUGETLB_PAGE && MMU
+	depends on MMU
 	help
 	  Allows the compaction of memory for the allocation of huge pages.