linux-ext4 - linux-next ext4 inode size 128 corrupted

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Date:   Mon, 17 Jul 2023 20:43:30 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Jeff Layton <jlayton@...nel.org>
cc:     Theodore Ts'o <tytso@....edu>,
        Christian Brauner <brauner@...nel.org>,
        Jan Kara <jack@...e.cz>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: linux-next ext4 inode size 128 corrupted

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