mm, vmpressure: pass-through notification support

By default, vmpressure events are not pass-through, i.e.  they propagate
up through the memcg hierarchy until an event notifier is found for any
threshold level.

This presents a difficulty when a thread waiting on a read(2) for a
vmpressure event cannot distinguish between local memory pressure and
memory pressure in a descendant memcg, especially when that thread may
not control the memcg hierarchy.

Consider a user-controlled child memcg with a smaller limit than a
top-level memcg controlled by the "Activity Manager" specified in
Documentation/cgroup-v1/memory.txt.  It may register for memory pressure
notification for descendant memcgs to make a policy decision: oom kill a
low priority job, increase the limit, decrease other limits, etc.  If it
registers for memory pressure notification on the top-level memcg, it
currently cannot distinguish between memory pressure in its own memcg or
a descendant memcg, which is user-controlled.

Conversely, if a user registers for memory pressure notification on
their own descendant memcg, the Activity Manager does not receive any
pressure notification for that child memcg hierarchy.  Vmpressure events
are not received for ancestor memcgs if the memcg experiencing pressure
have notifiers registered, perhaps outside the knowledge of the thread
waiting on read(2) at the top level.

Both of these are consequences of vmpressure notification not being
pass-through.

This implements a pass-through behavior for vmpressure events.  When
writing to control.event_control, vmpressure event handlers may
optionally specify a mode.  There are two new modes:

 - "hierarchy": always propagate memory pressure events up the hierarchy
   regardless if descendant memcgs have their own notifiers registered,
   and

 - "local": only receive notifications when the memcg for which the
   event is registered experiences memory pressure.

Of course, processes may register for one notification of "low,local",
for example, and another for "low".

If no mode is specified, the current behavior is maintained for
backwards compatibility.

See the change to Documentation/cgroup-v1/memory.txt for full
specification.

[dan.carpenter@oracle.com: free the same pointer we allocated]
  Link: http://lkml.kernel.org/r/20170613191820.GA20003@elgon.mountain
Link: http://lkml.kernel.org/r/alpine.DEB.2.10.1705311421320.8946@chino.kir.corp.google.com


Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Anton Vorontsov <anton@enomsg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>