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SubjectRe: 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@redhat.com> wrote:
> > On Tue, Nov 14, 2017 at 8:08 AM, Michael Kerrisk (man-pages)
> > <mtk.manpages@gmail.com> wrote:
> >> Hi Miklos, Ram
> >>
> >> Thanks for your comments. A question below.
> >>
> >> On 13 November 2017 at 09:11, Miklos Szeredi <mszeredi@redhat.com> wrote:
> >>> On Mon, Nov 13, 2017 at 8:55 AM, Ram Pai <linuxram@us.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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