pagemap: avoid splitting thp when reading /proc/pid/pagemap

pagemap: avoid splitting thp when reading /proc/pid/pagemap

Thp split is not necessary if we explicitly check whether pmds are mapping
thps or not.  This patch introduces this check and adds code to generate
pagemap entries for pmds mapping thps, which results in less performance
impact of pagemap on thp.

Signed-off-by: Naoya Horiguchi <[email protected]>
Reviewed-by: Andi Kleen <[email protected]>
Reviewed-by: KAMEZAWA Hiroyuki <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Wu Fengguang <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>