linux-ext4 - Re: Issue with ext4 filesystem corruption when writing to a file after disk exhaustion

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Message-ID: <CAJxJ_jgg0H=+JLSjc6SNwa5tiDhWjTunNPE2V1SP-v9_O8oCqw@mail.gmail.com>
Date: Tue, 15 Jul 2025 09:27:01 +0800
From: Jiany Wu <wujianyue000@...il.com>
To: "Theodore Ts'o" <tytso@....edu>
Cc: "Darrick J. Wong" <djwong@...nel.org>, yi.zhang@...wei.com, jack@...e.cz, 
	linux-ext4@...r.kernel.org
Subject: Re: Issue with ext4 filesystem corruption when writing to a file
 after disk exhaustion

Hello, Ted,

Thanks indeed for the clarification, it is clear now.
OK, if using a loopback mounted image on a disk, underlying file
system full then the block device will have I/O error.
This loopback mount belongs to a third party common config. I'll
fallocate lower disk space to not exhaust disk as a work around
firstly.
Thanks again for the help:)

Best regards,
Jianyue Wu


On Mon, Jul 14, 2025 at 9:09 PM Theodore Ts'o <tytso@....edu> wrote:
>
> On Mon, Jul 14, 2025 at 12:37:21PM +0800, Jiany Wu wrote:
> > Hello, Ted,
> >
> > Good day, thanks indeed for the clarification~
> > Yes, previously tried to mount a specific ext4 disk-img to /var/log,
> > with /dev/loop1 device, and rsyslogd will write to /var/log/syslog.
> > When /tmp directory exhaust manually via fallocate, / dir will be also
> > occupied as 100%, and rsyslog write errors in /dev/loop1 happen, later
> > mount as read-only. Different from the early scenario, but this
> > scenario is not easy to reproduce.
> > Tried updating the test case, not fallocate all spaces in disk, now
> > alloc 95%, everything is normal now, no related error prints anymore.
> > It is confirmed errors are caused by disk exhaust.
> > I think the main hesitation part is whether fallocate is allowed to
> > use the whole disk space.
>
> The fallocate system call is allowed to use the whole space on the
> *file system*.  But it doesn't know about how much free space a
> thin-provisioned device's underlying storage is available.  If you are
> using a loopback mounted image on a disk, if the underlying file
> system on the disk fills up then the block device will have I/O errors
> --- and then the file system on the loop device will run into
> problems, either data loss or metadata corruption.
>
> So this is working as intended.  If you don't want this, either don't
> use a loopback mount with a sparse file; either use fallocate when
> creating the image file, or don't use a loopback mount.
>
>                                 - Ted