linux-next ext4 inode size 128 corrupted

[Hugh Dickins]

Hi Jeff,

I've been unable to run my kernel builds on ext4 on loop0 on tmpfs
swapping load on linux-next recently, on one machine: various kinds
of havoc, most common symptoms being ext4_find_dest_de:2107 errors,
systemd-journald errors, segfaults.  But no problem observed running
on a more recent installation.

Bisected yesterday to 979492850abd ("ext4: convert to ctime accessor
functions").

I've mostly averted my eyes from the EXT4_INODE macro changes there,
but I think that's where the problem lies.  Reading the comment in
fs/ext4/ext4.h above EXT4_FITS_IN_INODE() led me to try "tune2fs -l"
and look at /etc/mke2fs.conf.  It's an old installation, its own
inodes are 256, but that old mke2fs.conf does default to 128 for small
FSes, and what I use for the load test is small.  Passing -I 256 to the
mkfs makes the problems go away.

(What's most alarming about the corruption is that it appears to extend
beyond just the throwaway test filesystem: segfaults on bash and libc.so
from the root filesystem.  But no permanent damage done there.)

One oddity I noticed in scrutinizing that commit, didn't help with
the issues above, but there's a hunk in ext4_rename() which changes
-	old.dir->i_ctime = old.dir->i_mtime = current_time(old.dir);
+	old.dir->i_mtime = inode_set_ctime_current(old.inode);

Hugh