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

[Michael Kerrisk (man-pages)]

Hi Miklos, Ram

Thanks for your comments. A question below.

On 13 November 2017 at 09:11, Miklos Szeredi <mszeredi@xxxxxxxxxx> wrote:
> 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".

Sorry -- do you mean the old description is correct, or my new
description (below)?

Cheers,

Michael


> 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
>>



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