[$] Filesystem metadata memory management

It is a good thing that strong coffee was served at the 2018 Linux Storage,Filesystem, and Memory-Management Summit; full awareness was required fromthe first session, in which Josef Bacik discussed some issues that havearisen in the interaction between filesystems and the memory-managementsubsystem. Filesystems cache a lot of data from files, but also a lot ofmetadata about those files. It turns out, though, that management of thecached metadata does not work as well as one might like.