Re: btrfs multiple mounts stacked on the same mount point

[Duncan <1i5t5.duncan@cox.net>]

Anand Jain posted on Tue, 11 Feb 2014 16:18:02 +0800 as excerpted:

> per mount(2)
> 
>   ---
>    multiple mounts can be stacked on the same mount point.
>   ---
> 
>   In this situation how could ioctl communicate (using mount point) with
>   each FS stacked on the same mount point ?
> 
>   BTW I don't understand the need for multiple mounts on the same mount
>   point ?

The most common case of multiple over-mounts is almost certainly the 
kernel's built-in rootfs in RAM, usually as an initramfs or initrd, with 
real-root often directly over-mounted the same mountpoint, tho it can 
also be mounted elsewhere on the initramfs.

The various union-filesystem solutions also directly use over-mounting, 
with the read-only mount often directly over-mounted with the writable 
but semi-transparent overlay, such that if a file hasn't changed from the 
read-only version on the under-mount, that's the version that gets used.

Otherwise, over-mounts generally obscure what's underneath them, making 
direct access to it impossible, unless that underneath filesystem is bind-
mounted elsewhere.

(Which is actually how I backup my root filesystem, using a bind-mount to 
mount it elsewhere for the backup, so for instance the /dev/console and 
/dev/null device nodes located directly on root get copied over to my 
backup root as well, instead of the devtmpfs content otherwise over-
mounted on /dev.)

That principle of over-mount obscuring what's beneath it should apply to 
the ioctls as well.  They will always communicate with the top mounted 
layers.  To communicate with anything underneath, the over-mounting 
layers will either need umounted, or (if it's not the exact same 
mountpoint, which might have been your point) a bind-mount of the under-
mount can be used.

But an over-mount obscuring under-mounts is how Linux (and I believe POSIX 
in general) normally works.  (The semi-transparent union-filesystem 
solutions mentioned above thus being exceptions with those exceptions 
being the hairy bits, that explaining the several implementations with 
their various limitations and bugs.)  So not being able to access under-
mounts is the normal state of affairs. =:^)

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
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