Thanks. In the context of btrfs, what is the critical need of this feature ? OR what is that it can't do without this feature ? Thanks, Anand On 02/12/14 04:57 AM, Duncan wrote:
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. =:^)
-- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
