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Date:   Thu, 8 Jul 2021 14:18:41 +0200
From:   Simon Richter <Simon.Richter@hogyros.de>
To:     linux-block@vger.kernel.org
Subject: fast creation and deletion of loop devices
Message-ID: <20210708121841.GA228715@psi5.com>
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Hi,

I have just used a crude loop to search for a partition on a block
device:

    for i in `seq 1 4096`
    do
        losetup -r -o ${i}k /dev/loop0 /dev/sda
        mount /dev/loop0 /mnt && break
        losetup -d /dev/loop0
    done

Creating the loop device fails sometimes, with

    losetup: /dev/sda: failed to set up loop device: Device or resource busy

and the kernel log contains a few instances of

    loop_set_status: loop0 () has still dirty pages (nrpages=5)

My machine is now in a state where I can call

    until losetup -d /dev/loop0; do :; done

and the loop just keeps running.

I'm slightly confused why a read-only loop device would have dirty pages
in the first place, and I suspect that there is a kernel bug here that
allows me to enter an inconsistent state through a race condition if I
just attach and detach loop devices fast enough.

   Simon