Discover FDs on lazily umounted filesystems in Linux

gistfile1.txt

There's no "right" way to do this, so...

When you lazily umount a filesystem in Linux, procfs changes /proc/pid/fd/* from absolute to relative paths. E.g. if you have a tail open on the FS:

[uid] 31624 0.0  0.0 100924  616 pts/86   Ss+  10:12   0:00 tail -f /mnt/test
$ readlink /proc/31624/fd/3
/mnt/test

Then lazily umount the FS:

$ umount -fl /mnt

$ readlink /proc/31624/fd/3
test

...Linux changes the absolute path to a relative one.  There are only three valid scenarios in which a symlink in /proc/[pid]/fd can point to a relative path:

1) The symlink points to a [socket] or [pipe]
2) The symlink points to "inotify"
3) The symlink points to a file on a lazily umounted FS.

So we can find all symlinks in /proc/*/fd, readlink them, get rid of the ones pointing to a socket, pipe, or inotify FD, get rid of ones pointing to absolute paths, and whatever is left is an open file descriptor for a lazily umounted filesystem.

Solution pasted below.

for lnk in $(find /proc/*/fd -type l 2>/dev/null | grep -v $$); do
    readlink ${lnk} | egrep -v "[\[/]|inotify" >/dev/null && 
        echo "lazily umounted file discovered at: ${lnk}"
done