linux-kernel - Weirdness with bind mounts and moving files from tmpfs

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Message-ID: <ZDic8fx1NQirB2+d@hoboy.vegasvil.org>
Date:   Thu, 13 Apr 2023 17:23:13 -0700
From:   Richard Cochran <richardcochran@...il.com>
To:     linux-kernel@...r.kernel.org
Subject: Weirdness with bind mounts and moving files from tmpfs

Dear list,

If a file is copied from tmpfs onto a bind mount, the contents update
as expected.  However if a file is _moved_ from tmpfs onto a bind
mount, the old content persists.

For example, my PC has a tmpfs on /run:

    # findmnt /run
    TARGET SOURCE FSTYPE OPTIONS
    /run   tmpfs  tmpfs  rw,nosuid,nodev,noexec,relatime,size=5737612k,mode=755,inode64

Set up a bind mounted file...

    # touch /opt/file
    # echo one > /home/file
    # mount -o bind /home/file /opt/file
    # cat /opt/file
    one

Coping a file onto the bind mount via the tmpfs on /run yields...

    # echo two > /run/file
    # cp /run/file /home/file
    # cat /opt/file
    two

So far, so good.  Now do the same, but with 'mv' instead of 'cp'...

    # echo three > /run/file
    # mv /run/file /home/file
    # cat /opt/file
    two

At this point, the contents of /opt/file are stuck forever with "two"...

    # echo three > /run/file
    # cp /run/file /home/file
    # cat /opt/file
    two
    # echo three > /home/file
    # cat /opt/file
    two

What is going on here?  Is this a bug or a feature?

Thanks,
Richard