Sigh. Always google before stracing ... google reveals this: https://bugzilla.redhat.com/show_bug.cgi?id=753882
which indicates that 1) the bug is more than 3 years old 2) its bitten hundreds of people, 3) pam_systemd is "working as designed" : it is pulling the user directory from the environment variable XDG_RUNTIME_DIR=/run/user/1000 (well, 500 in my case) 4) fedora is in a state of denial. Probably need to close this bug, and instead do to things: 1) fix mate-stettings daemon so is doesn't blow through 16GB ram in 5 minutes 2) track down who set the XDG_RUNTIME_DIR env var
