khugepaged: allow to collapse PTE-mapped compound pages

khugepaged: allow to collapse PTE-mapped compound pages

We can collapse PTE-mapped compound pages.  We only need to avoid handling
them more than once: lock/unlock page only once if it's present in the PMD
range multiple times as it handled on compound level.  The same goes for
LRU isolation and putback.

Signed-off-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Tested-by: Zi Yan <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Acked-by: Yang Shi <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Ralph Campbell <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>