FYI, we noticed the following commit:

commit: 81e3bb1b6f6d84d47e773f34a14b0955e528c6eb ("mnt: Tuck mounts under others instead of creating shadow/side mounts.")
https://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git for-testing

in testcase: boot

on test machine: qemu-system-x86_64 -enable-kvm -cpu host -smp 2 -m 1G

caused below changes:


+------------------------------------------------------------------+------------+------------+
|                                                                  | f84df2a6f2 | 81e3bb1b6f |
+------------------------------------------------------------------+------------+------------+
| boot_successes                                                   | 14         | 3          |
| boot_failures                                                    | 0          | 13         |
| BUG:sleeping_function_called_from_invalid_context_at_fs/dcache.c | 0          | 12         |
| calltrace:SyS_mount                                              | 0          | 12         |
| invoked_oom-killer:gfp_mask=0x                                   | 0          | 1          |
| Mem-Info                                                         | 0          | 1          |
+------------------------------------------------------------------+------------+------------+



[   11.335778] rc.local[3455]: PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/lkp/lkp/src/bin
         Starting Login Service...
[   12.095031] random: fast init done
[   12.107647] BUG: sleeping function called from invalid context at fs/dcache.c:757
[   12.109075] in_atomic(): 1, irqs_disabled(): 0, pid: 3475, name: mount
[   12.109967] 4 locks held by mount/3475:
[   12.110636]  #0: 
[   12.110834]  (
&sb->s_type->i_mutex_key
[   12.111598] #2
){++++++}
[   12.112265] , at: 
[   12.112834] [<ffffffff8121f940>] lock_mount+0x2f/0xf6
[   12.113591]  #1: 
[   12.113786]  (
namespace_sem
[   12.122117] ){++++++}
, at: 
[   12.122794] [<ffffffff8121f96c>] lock_mount+0x5b/0xf6
[   12.123608]  #2: 
[   12.123803]  (
mount_lock
[   12.124468] ){+.+...}
, at: 
[   12.125163] [<ffffffff812206b5>] attach_recursive_mnt+0xb8/0x2c1
[   12.126030]  #3: 
[   12.126242]  (
mount_lock
[   12.126898] #2
){+.+...}
[   12.127565] , at: 
[   12.128142] [<ffffffff81220912>] graft_tree+0x54/0x56
[   12.128894] CPU: 0 PID: 3475 Comm: mount Not tainted 4.9.0-rc6-00005-g81e3bb1 #1
[   12.130158] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Debian-1.8.2-1 04/01/2014
[   12.131500]  ffffc90002b17d18 ffffffff8158eef7 ffff88003e1cc040 00000000000002f5
[   12.132936]  ffffc90002b17d40 ffffffff810cbb24 ffffffff82226d44 00000000000002f5
[   12.134378]  0000000000000000 ffffc90002b17d68 ffffffff810cbbae ffff88003376a7b8
[   12.135825] Call Trace:
[   12.136385]  [<ffffffff8158eef7>] dump_stack+0x86/0xc0
[   12.137151]  [<ffffffff810cbb24>] ___might_sleep+0x1fa/0x20d
[   12.137941]  [<ffffffff810cbbae>] __might_sleep+0x77/0x7e
[   12.138723]  [<ffffffff81214f74>] dput+0x38/0x357
[   12.139457]  [<ffffffff8120853a>] path_put+0x16/0x21
[   12.140204]  [<ffffffff8121fbfa>] mnt_change_mountpoint+0xc2/0xcf
[   12.141019]  [<ffffffff812207e8>] attach_recursive_mnt+0x1eb/0x2c1
[   12.141853]  [<ffffffff81220912>] graft_tree+0x54/0x56
[   12.142613]  [<ffffffff812209bf>] do_add_mount+0xab/0xca
[   12.143393]  [<ffffffff81221665>] do_mount+0xaa7/0xb06
[   12.144162]  [<ffffffff81220ab8>] ? copy_mount_options+0x2e/0x117
[   12.144975]  [<ffffffff81221906>] SyS_mount+0x77/0x9f
[   12.145735]  [<ffffffff81d177c1>] entry_SYSCALL_64_fastpath+0x1f/0xc2


To reproduce:

        git clone git://git.kernel.org/pub/scm/linux/kernel/git/wfg/lkp-tests.git
        cd lkp-tests
        bin/lkp qemu -k <bzImage> job-script  # job-script is attached in this email



Thanks,
Kernel Test Robot