Ccing linux-fsdevel. To catch up if you are just tuning in, the
original problem is that "mount /run" mounts a new instance of tmpfs
over top of the existing one, hiding the existing files, rather than
reporting that it is already mounted. The question is, is this a bug in
mount, or the kernel?
On 6/22/2015 10:15 AM, Karel Zak wrote:
# strace -e mount mount /boot
mount("/dev/sda2", "/boot", "ext4", MS_MGC_VAL, NULL) = -1 EBUSY
(Device or resource busy)
Why on earth is the kernel still returning EBUSY here? It *does*
support mounting the same block device multiple times these days so it
should not be doing this. It looks like it has some check to see if
that device is already mounted somewhere in the current filesystem
namespace and returns EBUSY if it is, otherwise, just bind mounts the
existing mount if it is mounted in a different filesystem namespace.
Not only does this check seem pointless, but it simply makes no sense at
all for any filesystem that isn't backed by a block device, such as
tmpfs, procfs, network filesystems, etc.
$ mount /tmp/proc/
mount: proc is already mounted or /tmp/proc busy
proc is already mounted on /proc
proc is already mounted on /tmp/proc
strace -e mount mount -t proc none /proc
mount("none", "/proc", "proc", MS_MGC_VAL, NULL) = -1 EBUSY (Device or
resource busy)
Now *what* is this nonsense? You can mount proc any time, anywhere you
want to. This EBUSY seems to be a special case hack that you only get
if you try to mount procfs inside procfs. You can mount any other fs
over top of /proc, and you can mount /proc over top of any other fs.
Why the one off check for mounting proc on top of proc? And is tmpfs
and any other virtual filesystem supposed to do this same check?
What if you really *want* to mount a new tmpfs over the old one? The
kernel shouldn't be denying that request ( really, same goes for proc ).
It therefore, should be the responsibility of mount ( single argument
form ) to notice when you have requested mounting of an already mounted
filesystem listed in /fstab, and it certainly should not be treating
/etc/mtab as if it were /etc/fstab and trying to mount the same thing a
second time; the single argument form of mount should only consult
fstab, not mtab too.
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