Improving documentation of parent-ID field in /proc/PID/mountinfo

[Michael Kerrisk (man-pages)]

Hello Ram,

Long ago (2.6.29) you added the /proc/PID/mountinfo file and
associated documentation in Documentation/filesystems/proc.txt. Later,
I pasted much of that documentation into the proc(5) manual page.

That documentation says of the second field in the file:

[[
This file contains lines of the form:

36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
(1)(2)(3)   (4)   (5)      (6)      (7)   (8) (9)   (10)         (11)

(1) mount ID:  unique identifier of the mount (may be reused after umount)
(2) parent ID:  ID of parent (or of self for the top of the mount tree)
...
]]

The last piece of the description of field (2) doesn't seem to be
correct, or is at least rather unclear. I take this to be saying that
that for the root mount point, /, field (2) will have the same value
as field (1). I never actually looked at this detail closely, but
Alexander pointed out that this is obviously not so, as one can
immediately verify:

$ grep '/ / ' /proc/$$/mountinfo
65 0 8:2 / / rw,relatime shared:1 - ext4 /dev/sda2 rw,seclabel,data=order

I dug around in the kernel source for a bit. I do not have an exact
handle on the details, but I can see roughly what is going on.
Internally, there seems to be one ("hidden") mount ID reserved to each
mount namespace, and that ID is the parent of the root mount point.

Looking through the (4.14) kernel source, mount IDs are allocated by
mnt_alloc_id() (in fs/namespace.c), which is in turn called by
alloc_vfsmnt() which is in turn called by clone_mnt().

A new mount namespace is created by the kernel function copy_mnt_ns()
(in fs/namespace.c, called by create_new_namespaces() in
kernel/nsproxy.c). The copy_mnt_ns() function calls copy_tree() (in
fs/namespace.c), and copy_tree() calls clone_mnt() in *two* places.
The first of these is the call that creates the "hidden" mount ID that
becomes the parent of the root mount point. (I verified this by
instrumenting the kernel with a few printk() calls to display the
IDs.)  The second place where copy_tree() calls clone_mnt() is in a
loop that replicates each of the mount points (including the root
mount point) in the source mount namespace.

With these details in mind, I propose to patch the man page to read as
below. Perhaps you have some corrections or improvements to suggest
for this text?

[[
              (2)  parent ID: the ID of the parent mount.  For  the  root
                   mount  point,  the  ID shown here is a hidden mount ID
                   associated with the mount namespace.  That ID is  disâ
                   tinct  from  any  of the IDs shown in field (1) of the
                   records shown in the  mountinfo  file,  and  does  not
                   appear in field (1) in the mountinfo file in any other
                   mount namespace.  (In  the  initial  mount  namespace,
                   this hidden ID has the value 0.)
]]

With best regards,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/