[$] Shrinking filesystem caches for dying control groups

In a followup to his earlier session on dying
control groups, Roman Gushchin wanted to talk about problems with the
shrinkers and filesystem caches in a combined filesystem and
memory-management session at the
2019 Linux Storage, Filesystem, and Memory-Management Summit (LSFMM).
Specifically, for control groups that share the same underlying filesystem,
the shrinkers are not able to reclaim memory from the VFS caches after a
control group dies, at least under slight to moderate memory pressure. He
wanted to discuss how to reclaim that memory without major performance
impacts.