The symptoms: I use Eclipse Luna on Ubuntu 14.04.
My workspace is quite old and contains many projects. At some moment, I could not save any file anymore. When clicking Ctrl + S, Eclipse was freezing forever.
The cure: I deleted the projects I was not working on anymore. Reducing the number of files managed in my workspace solved the problem.
Notice this could also be due to an OS limit.
Or it could be due to the number of files managed by the workspace. In any case, clean it up. Save actions should work again.