Re: Warning: empty root dentry name after lazy mount removal

[Omar Sandoval <osandov@osandov.com>]

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On Fri, May 15, 2015 at 08:58:20PM +0200, Ivan Delalande wrote:
> Hi,
> 
> Since commit 8ed936b "vfs: Lazily remove mounts on unlinked files and
> directories.", I’m seeing a new warning from prepend_path() at
> fs/dcache.c:2937 saying:
> 
> 	Root dentry has weird name <>
> 
> Our particular occurrence happens because we are using an older version
> of iproute (< v3.10, commit bcb9d40, but this also works with the master
> and just reverting that commit), and the following scenario:
> 
> 	term1# ip netns add foo
> 	term1# ip netns exec foo sh
> 	term1# ls -l /proc/self/fd/4
> 	lr-x------ 1 root root 64 May 15 11:02 /proc/self/fd/4 -> /var/run/netns/foo
> 	term2# ip netns del foo
> 	term1# ls -l /proc/self/fd/4
> 	[dmesg] WARNING: [...]
> 	[dmesg] Root dentry has weird name <>
> 	lr-x------ 1 root root 64 May 15 11:04 /proc/self/fd/4 -> /
> 
> Relevant moutinfo chunks before the `ip netns del`:
> 
> 	29 0 8:2 / / rw,relatime - ext4 /dev/sda2 rw,data=ordered
> 	82 29 0:3 / /var/run/netns/foo rw,nosuid,nodev,noexec,relatime - proc proc rw
> 
> What I could observe is that after the namespace deletion, that mount 82
> get isolated (mnt->parent = mnt), and so prepend_path considers it as
> the global root. But the name of that dentry has d_name.len == 0, which
> triggers the WARN in that function.
> 
> I’m still struggling to understand all the relations between the VFS
> structures and at which point they get initialized precisely. Do you
> have any idea how to fix this problem?
> 

I'm attaching a minimal script that reproduces this on 4.1-rc4. I'm
taking a look, but Eric or Al will probably figure this out before I get
the chance :)

-- 
Omar

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#!/bin/sh

set -e

MNT=testmnt

# Create a new namespace and bind-mount it to keep it alive.
(
touch "$MNT"
unshare --net mount --bind /proc/self/ns/net "$MNT"
)

rm -f barrier && mkfifo barrier

# Hold a reference while the namespace gets unmounted.
(
exec 4<"$MNT"
ls -l /proc/self/fd
cat < barrier
ls -l /proc/self/fd
) &

# Unmount the namespace while there's still a reference.
cat /dev/null > barrier
umount --lazy "$MNT"
wait