Quotas for tmpfs

The classic Unix system which is used by multiple persons, each with a unique userid, are very rare now. On a system like that, a per-userid quota would make sense, though, to prevent a malicious user from denying service to others.

But multiuser systems today typically have multiple persons acting through the same userid (which owns a server process). On those, per-userid quota isn't much use.

Besides malicious attack, another great use of tmpfs space limitation is to limit the risk of an inadvertent runaway process. A per-process resource limit is a sensible way to do that.

Reserving space for root is a particularly ham-handed way protect system integrity, since root isn't all that special. It often uses space on behalf of unprivileged unimportant users, for one thing. And other userids are often critical to the system (there's no point in all those system management things running if the web server can't, after all).

There already is a great facility for protecting a process from another process' indiscriminate use of tmpfs space: make a tmpfs filesystem just for him. Any serious program ought either to respect the TMPDIR environment variable or provide some other means of directing the location of temporary files.