linux-ext4 - Bug: Buffer I/O error with an ext4 filesystem inside a file

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Date:   Wed, 1 Feb 2023 10:37:47 +0000
From:   Etienne Schmidt OSS <Etienne.Schmidt-oss@...dmueller.com>
To:     "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Bug: Buffer I/O error with an ext4 filesystem inside a file

Hello everyone!

I have tried to allocate disk space for a service. To do this I created a file with fixed disk usage and created an ext4 file system in it. When I mount this file the mount point should be reserved space but something went wrong. With full memory I get a buffer I/O error.

Observation
==========
I created an ext4 filesystem inside a file (ext4 partition below) and mounted it (e.g. in /var/persistent/reserved). Then I fill up the underlying file system. There is enough space inside the file but when I write a file to it the journal gives a "buffer I/O error".  

Steps to Reproduce
================
The following steps reproduce these bug. I execute them as root user.

Preparation:
1.	Create a file with fix disk usage:
	`fallocate -l 32M /var/reserved.ext4`
2.	Create a ext4 filesystem inside it:
	`mkfs.ext4 /var/reserved.ext4`
3.	Create the mountpoint:
	`mkdir /var/reserved/`
4.	Mount the file:
	`mount /var/reserved.ext4 /var/reserved/`
5.	(Optional) Check the filesystem with fsck.

Now the reserved storage works fine!

The Bug:
1.	Fill up the underlying filesystem:
	`fallocate -l 100G /var/very_big_file`
2.	Write into the reserved storage:
	`echo "Test" > /var/reserved/test_file_1`
	or anything else.
3.	The file is written but the journal shows the following error:

	```
	May 03 08:31:42 ucm kernel: loop: Write error at byte offset 8913920, length 1024.
	May 03 08:31:42 ucm kernel: blk_update_request: I/O error, dev loop0, sector 17410 op 0x1:(WRITE) flags 0x0 phys_seg 1 prio class 0
	May 03 08:31:42 ucm kernel: EXT4-fs warning (device loop0): ext4_end_bio:344: I/O error 10 writing to inode 12 starting block 8706)
	May 03 08:31:42 ucm kernel: Buffer I/O error on device loop0, logical block 8705
	May 03 08:31:42 ucm kernel: JBD2: Detected IO errors while flushing file data on loop0-8
	```

Quick fix
=======

Allocate a larger space to the file AFTER creating the ext4 file system.
E.g after step 2. within the reserved space preparation: `fallocate -l 33M /var/reserved.ext4`

Is this a bug or is my use wrong?

Best regards

Etienne Schmidt