vmscan: memcg: always use swappiness of the reclaimed memcg

vmscan: memcg: always use swappiness of the reclaimed memcg

Memory reclaim always uses swappiness of the reclaim target memcg (origin
of the memory pressure) or vm_swappiness for global memory reclaim.  This
behavior was consistent (except for difference between global and hard
limit reclaim) because swappiness was enforced to be consistent within
each memcg hierarchy.

After "mm: memcontrol: remove hierarchy restrictions for swappiness and
oom_control" each memcg can have its own swappiness independent of
hierarchical parents, though, so the consistency guarantee is gone.  This
can lead to an unexpected behavior.  Say that a group is explicitly
configured to not swapout by memory.swappiness=0 but its memory gets
swapped out anyway when the memory pressure comes from its parent with a
It is also unexpected that the knob is meaningless without setting the
hard limit which would trigger the reclaim and enforce the swappiness.
There are setups where the hard limit is configured higher in the
hierarchy by an administrator and children groups are under control of
somebody else who is interested in the swapout behavior but not
necessarily about the memory limit.

From a semantic point of view swappiness is an attribute defining anon vs.
 file proportional scanning of LRU which is memcg specific (unlike charges
which are propagated up the hierarchy) so it should be applied to the
particular memcg's LRU regardless where the memory pressure comes from.

This patch removes vmscan_swappiness() and stores the swappiness into the
scan_control structure.  mem_cgroup_swappiness is then used to provide the
correct value before shrink_lruvec is called.  The global vm_swappiness is
used for the root memcg.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>