Commit "new primitive: discard_new_inode()" introduces a problem

[Kirill Tkhai]

Hi, Al,

commit 22dc9a1682722fda8603991595528b87e90436f6
Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Date:   Thu Jun 28 15:53:17 2018 -0400

    new primitive: discard_new_inode()

from current linux-next introduces the following problem.
It reproduces 100% in my test VM:

# cat /etc/mtab 
/dev/sda1 / ext4 rw 0 0
proc /proc proc rw 0 0
sysfs /sys sysfs rw 0 0
devpts /dev/pts devpts rw,gid=5,mode=620 0 0
tmpfs /dev/shm tmpfs rw,noexec,nosuid,nodev 0 0
# echo 3 > /proc/sys/vm/drop_caches 
# cat /etc/mtab 
cat: /etc/mtab: Cannot allocate memory

Though:
# free
             total       used       free     shared    buffers     cached
Mem:       4047432      99988    3947444          0       1340      49148
-/+ buffers/cache:      49500    3997932
Swap:            0          0          0

Kirill