proc: export idle flag via kpageflags

proc: export idle flag via kpageflags

As noted by Minchan, a benefit of reading idle flag from /proc/kpageflags
is that one can easily filter dirty and/or unevictable pages while
estimating the size of unused memory.

Note that idle flag read from /proc/kpageflags may be stale in case the
page was accessed via a PTE, because it would be too costly to iterate
over all page mappings on each /proc/kpageflags read to provide an
up-to-date value.  To make sure the flag is up-to-date one has to read
/sys/kernel/mm/page_idle/bitmap first.

Signed-off-by: Vladimir Davydov <[email protected]>
Reviewed-by: Andres Lagar-Cavilla <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Raghavendra K T <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>