memcg: Slow down swap allocation as the available space gets depleted

Tejun describes the problem as follows:

When swap runs out, there's an abrupt change in system behavior -
the anonymous memory suddenly becomes unmanageable which readily
breaks any sort of memory isolation and can bring down the whole
system. To avoid that, oomd [1] monitors free swap space and triggers
kills when it drops below the specific threshold (e.g. 15%).

While this works, it's far from ideal:
 - Depending on IO performance and total swap size, a given
   headroom might not be enough or too much.
 - oomd has to monitor swap depletion in addition to the usual
   pressure metrics and it currently doesn't consider memory.swap.max.

Solve this by adapting parts of the approach that memory.high uses -
slow down allocation as the resource gets depleted turning the
depletion behavior from abrupt cliff one to gradual degradation
observable through memory pressure metric.

[1] https://github.com/facebookincubator/oomd

v5: https://lore.kernel.org/linux-mm/20200521002411.3963032-1...
v4: https://lore.kernel.org/linux-mm/20200519171938.3569605-1...
v3: https://lore.kernel.org/linux-mm/20200515202027.3217470-1...
v2: https://lore.kernel.org/linux-mm/20200511225516.2431921-1...
v1: https://lore.kernel.org/linux-mm/20200417010617.927266-1-...

Jakub Kicinski (4):
  mm: prepare for swap over-high accounting and penalty calculation
  mm: move penalty delay clamping out of calculate_high_delay()
  mm: move cgroup high memory limit setting into struct page_counter
  mm: automatically penalize tasks with high swap use

 Documentation/admin-guide/cgroup-v2.rst |  20 +++
 include/linux/memcontrol.h              |   4 +-
 include/linux/page_counter.h            |   8 ++
 mm/memcontrol.c                         | 177 ++++++++++++++++++------
 4 files changed, 160 insertions(+), 49 deletions(-)

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2.25.4