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

[Miklos Szeredi]

On Mon, Nov 13, 2017 at 8:55 AM, Ram Pai <linuxram@xxxxxxxxxx> wrote:
> 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.

The hidden one is the initramfs, I believe.  That's the root of the
mount namespace, and the when a namespace is cloned, the tree is
copied from the namespace root.

It is "hidden" because no process has its root there.  Note the
difference between namespace root and process root: the first is the
real root of the mount tree and is unchangeable, the second is
pointing to some place in a mount tree and can be changed (chroot).

So there's nothing special in this rootfs, it is just hidden because
it's not the root of any task.

The description of  field (2) is correct, it just does not make it
clear what it means by "root".

Thanks,
Miklos

>
>>
>> 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.)
>
> It captures the current semantics correctly.
>
> RP
>