[$] Shrinking filesystem caches for dying control groups

In a followup to his earlier session on dyingcontrol groups, Roman Gushchin wanted to talk about problems with theshrinkers and filesystem caches in a combined filesystem andmemory-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 acontrol group dies, at least under slight to moderate memory pressure. Hewanted to discuss how to reclaim that memory without major performanceimpacts.