[$] 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.