mm: workingset: eviction buckets for bigmem/lowbit machines

For per-cgroup thrash detection, we need to store the memcg ID inside
the radix tree cookie as well.  However, on 32 bit that doesn't leave
enough bits for the eviction timestamp to cover the necessary range of
recently evicted pages.  The radix tree entry would look like this:

[ RADIX_TREE_EXCEPTIONAL(2) | ZONEID(2) | MEMCGID(16) | EVICTION(12) ]

12 bits means 4096 pages, means 16M worth of recently evicted pages.
But refaults are actionable up to distances covering half of memory.  To
not miss refaults, we have to stretch out the range at the cost of how
precisely we can tell when a page was evicted.  This way we can shave
off lower bits from the eviction timestamp until the necessary range is
covered.  E.g.  grouping evictions into 1M buckets (256 pages) will
stretch the longest representable refault distance to 4G.

This patch implements eviction buckets that are automatically sized
according to the available bits and the necessary refault range, in
preparation for per-cgroup thrash detection.

The maximum actionable distance is currently half of memory, but to
support memory hotplug of up to 200% of boot-time memory, we size the
buckets to cover double the distance.  Beyond that, thrashing won't be
detectable anymore.

During boot, the kernel will print out the exact parameters, like so:

  [    0.113929] workingset: timestamp_bits=12 max_order=18 bucket_order=6

In this example, there are 12 radix entry bits available for the
eviction timestamp, to cover a maximum distance of 2^18 pages (this is a
1G machine).  Consequently, evictions must be grouped into buckets of
2^6 pages, or 256K.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>