On Mon, Nov 13, 2017 at 07:02:21AM +0100, Michael Kerrisk (man-pages) wrote:
> 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.
We used to report that mount, ones upon a time. Something has changed
the behavior since then and its not reported any more, thus making it
hidden.
>
> 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 dis‐
> 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.)
It captures the current semantics correctly.
RP
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