mm: add new KSM process and sysfs knobs

mm: add new KSM process and sysfs knobs

This adds the general_profit KSM sysfs knob and the process profit metric
and process merge type knobs to ksm_stat.

1) split off pages_volatile function

   This splits off the pages_volatile function.  The next patch will
   use this function.

2) expose general_profit metric

   The documentation mentions a general profit metric, however this
   metric is not calculated.  In addition the formula depends on the size
   of internal structures, which makes it more difficult for an
   administrator to make the calculation.  Adding the metric for a better
   user experience.

3) document general_profit sysfs knob

4) calculate ksm process profit metric

   The ksm documentation mentions the process profit metric and how to
   calculate it.  This adds the calculation of the metric.

5) add ksm_merge_type() function

   This adds the ksm_merge_type function.  The function returns the
   merge type for the process.  For madvise it returns "madvise", for
   prctl it returns "process" and otherwise it returns "none".

6) mm: expose ksm process profit metric and merge type in ksm_stat

   This exposes the ksm process profit metric in /proc/<pid>/ksm_stat.
   The name of the value is ksm_merge_type.  The documentation mentions
   the formula for the ksm process profit metric, however it does not
   calculate it.  In addition the formula depends on the size of internal
   structures.  So it makes sense to expose it.

7) document new procfs ksm knobs

Link: https://lkml.kernel.org/r/20230224044000.3084046-3-shr@devkernel.io
Link: https://lkml.kernel.org/r/20230310182851.2579138-3-shr@devkernel.io
Signed-off-by: Stefan Roesch <shr@devkernel.io>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>