Weirdness with bind mounts and moving files from tmpfs

Weirdness with bind mounts and moving files from tmpfs

[Richard Cochran]

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

Re: Weirdness with bind mounts and moving files from tmpfs

[Al Viro]

On Thu, Apr 13, 2023 at 05:23:13PM -0700, Richard Cochran wrote:

> 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?

strace and compare the traces...  cp(1) opens the existing
target, truncates and writes to it; mv(1) tries rename(),
falling back to unlink()+creat()+write() in case rename()
failed with -EXDEV.

The same behaviour can be seen without any mount(2) use:

; echo x > a
; ln a b
; echo y > c
; cat a
x
; cat b
x
; cat c
y
; cp c a
; cat b
y
; echo x > a
; cat a
x
; cat b
x
; mv c a
; cat a
y
; cat b
x
;

Exact same underlying cause - writing down what happens
step-by-step might be enlightening...