emacs fails to backup

[Andries . Brouwer]

Usually, emacs will save a backup of a file that was edited.
There are good and bad sides to this, but that is what
emacs does by default, and one comes to depend on it.
Today I lost some time because emacs failed to make a backup.

Normally my Linux trees are mounted on /linux, or /mnt or
other directories. Today, on this computer, it was /tmp/linux.
Edit a file, with the purpose of sending off the resulting
diff after
        diff -u file~ file
but there was no file~.

Investigation of the info file shows that, indeed, if one is
deep below /mnt then things are saved, but if one is deep below
/tmp then things are not saved. Bad. Very bad context dependence.

This means that the default version of emacs will make backup files
when you don't need them, but that you cannot rely on getting the
backup when you depend on it. The worst of all worlds.

Andries