We use refault information to determine whether the cache workingset is
stable or transitioning, and dynamically adjust the inactive:active file
LRU ratio so as to maximize protection from one-off cache during stable
periods, and minimize IO during transitions.

With cgroups and their nested LRU lists, we currently don't do this
correctly.  While recursive cgroup reclaim establishes a relative LRU
order among the pages of all involved cgroups, refaults only affect the
local LRU order in the cgroup in which they are occuring.  As a result,
cache transitions can take longer in a cgrouped system as the active pages
of sibling cgroups aren't challenged when they should be.

[ Right now, this is somewhat theoretical, because the siblings, under
  continued regular reclaim pressure, should eventually run out of
  inactive pages - and since inactive:active *size* balancing is also
  done on a cgroup-local level, we will challenge the active pages
  eventually in most cases. But the next patch will move that relative
  size enforcement to the reclaim root as well, and then this patch
  here will be necessary to propagate refault pressure to siblings. ]

This patch moves refault detection to the root of reclaim.  Instead of
remembering the cgroup owner of an evicted page, remember the cgroup that
caused the reclaim to happen.  When refaults later occur, they'll
correctly influence the cross-cgroup LRU order that reclaim follows.

I.e.  if global reclaim kicked out pages in some subgroup A/B/C, the
refault of those pages will challenge the global LRU order, and not just
the local order down inside C.

[hannes@cmpxchg.org:  use page_memcg() instead of another lookup]
  Link: http://lkml.kernel.org/r/20191115160722.GA309754@cmpxchg.org
Link: http://lkml.kernel.org/r/20191107205334.158354-3-hannes@cmpxchg.orgSigned-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>