mm: workingset: size shadow nodes lru basing on file cache size

A page is activated on refault if the refault distance stored in the
corresponding shadow entry is less than the number of active file pages.
Since active file pages can't occupy more than half memory, we assume
that the maximal effective refault distance can't be greater than half
the number of present pages and size the shadow nodes lru list
appropriately.  Generally speaking, this assumption is correct, but it
can result in wasting a considerable chunk of memory on stale shadow
nodes in case the portion of file pages is small, e.g.  if a workload
mostly uses anonymous memory.

To sort this out, we need to compute the size of shadow nodes lru basing
not on the maximal possible, but the current size of file cache.  We
could take the size of active file lru for the maximal refault distance,
but active lru is pretty unstable - it can shrink dramatically at
runtime possibly disrupting workingset detection logic.

Instead we assume that the maximal refault distance equals half the
total number of file cache pages.  This will protect us against active
file lru size fluctuations while still being correct, because size of
active lru is normally maintained lower than size of inactive lru.

Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>