Date:   Mon, 20 Nov 2017 19:19:47 -0800
From:   Ram Pai <linuxram@...ibm.com>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        lkml <linux-kernel@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        Alexander Eder <alexander.eder@...man.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: Improving documentation of parent-ID field in /proc/PID/mountinfo

On Mon, Nov 20, 2017 at 10:07:29AM +0100, Michael Kerrisk (man-pages) wrote:
> Hi Miklos,
> 
> Sorry for the slow follow-up.
> 
> On 14 November 2017 at 17:16, Miklos Szeredi <mszeredi@...hat.com> wrote:
> > On Tue, Nov 14, 2017 at 8:08 AM, Michael Kerrisk (man-pages)
> > <mtk.manpages@...il.com> wrote:
> >> Hi Miklos, Ram
> >>
> >> Thanks for your comments. A question below.
> >>
> >> On 13 November 2017 at 09:11, Miklos Szeredi <mszeredi@...hat.com> wrote:
> >>> On Mon, Nov 13, 2017 at 8:55 AM, Ram Pai <linuxram@...ibm.com> 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)?
> >
> > Well, both are correct, yours just describes the same thing at the
> > higher level.  But I think rootfs is an implementation detail, so is
> > the fact that it gets a zero mount ID, so I think the original
> > description better captures the essence of the interface.  Except it
> > needs to clarify what "top of the mount tree" means.  It doesn't mean
> > current process's root, rather it means the root of the mount tree in
> > the current mount namespace.
> 
> Thanks for the further info.
> 
> But, the problem is that the existing description is at best misleading:
> 
>               (2)  parent  ID: the ID of the parent mount (or of self for
>                    the top of the mount tree).
> 
> That implies that we'll find one line in the list where field 1 and
> field 2 are the same. But we don't, because the mountns rootfs entry
> is not shown in mountinfo. On top of that, the reader is left
> confused, because when they look at mountinfo, they see one entry
> where the parent-ID doesn't exist in the list. So, something more than
> the current text is required. After digging around in the kernel
> source and noticing that chroot() will also cause this scenario, and
> taking into account your comments, I revised the text to:
> 
>               (2)  parent ID: the ID of the parent mount (or of self  for
>                    the root of this mount namespace's mount tree).
> 
>                    If  the  parent mount point lies outside the process's
>                    root directory (see  chroot(2)),  the  ID  shown  here
>                    won't  have  a corresponding record in mountinfo whose
>                    mount ID  (field  1)  matches  this  parent  mount  ID
>                    (because  mount  points that lie outside the process's
>                    root directory are not shown in mountinfo).  As a spe‐
>                    cial  case  of  this  point,  the process's root mount
>                    point may have  a  parent  mount  (for  the  initramfs
>                    filesystem)  that  lies  outside  the  process's  root
>                    directory, and an entry for that mount point will  not
>                    appear in mountinfo.
> 
> How does that seem?

yes. captures it well.
RP

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