linux-ext4 - [Bug 213627] Fail to read block descriptors data of ext4 filesystem

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Message-ID: <bug-213627-13602-6yfWtwuOft@https.bugzilla.kernel.org/>
Date:   Thu, 01 Jul 2021 00:24:04 +0000
From:   bugzilla-daemon@...zilla.kernel.org
To:     linux-ext4@...r.kernel.org
Subject: [Bug 213627] Fail to read block descriptors data of ext4 filesystem

https://bugzilla.kernel.org/show_bug.cgi?id=213627

Theodore Tso (tytso@....edu) changed:

           What    |Removed                     |Added
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                 CC|                            |tytso@....edu

--- Comment #2 from Theodore Tso (tytso@....edu) ---
I'm guessing that it's your snapshot driver which is buggy.   Certainly, if you
take a snapshot using LVM, things work fine.  e.g.:

# mke2fs -t ext4 /dev/cwcc-wg/scratch
# mount -t ext4 /dev/cwcc-wg/scratch /mnt
# cp -r /etc /mnt
# lvcreate --snapshot -n snap -L 5G cwcc-wg/scratch
# e2fsck -fn /dev/cwcc-wg/snap

You can see everything that has changed via a command such as "git log
v5.0..v5.3 block fs/ext4".    In terms of what might be a relevant change,
without understanding how your snapshot driver works, your guess is probably
going to be better than mine --- since you have access to your snapshot driver
and know how it works.

When you say that your driver "bypasses read/write calls to system block
driver", I'm not 100% sure how it works, but at a guess, some things I'd look
at are: (a) ext4 uses the buffer cache to read/write metadata blocks.   Maybe
your driver isn't properly intercepting buffer cache reads/writes?    (b) Ext4
at mount time reads the superblock via the buffer cache with the block size set
to 1k; and then after it determines the block size of the file system (say,
4k), it switches the block size of the buffer cache to the block size of the
file system.    Ext[234] has been doing this for decades, but depending on how
your snapshot driver is working, perhaps there is some change in the how the
buffer cache works which is confusing your driver.

Sorry I can't help more.

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