"Any filesystem could conceivably implement this functionality, but, in practice, it only makes sense for a RAM-based filesystem like tmpfs, so it is only implemented there."
So, for the stated example, the web browser would have to:
1. Find a mounted ram-based filesystem it had access to.
2. Create a file for the cache.
3. Presumably mmap() the file so it can be accessed conveniently.
Also, how would the application know when an area had been reclaimed? Canaries? What if an application tried to access an object that had only partially been reclaimed - say a single 4k page had innocuously been taken from a huge blob?