Re: [PATCH] man/man2/move_mount.2: document EINVAL on multiple instances

[Askar Safin <safinaskar@gmail.com>]

Okay, I spent some more time researching this.

By default move_mount should work in your case.

But if we try to move mount, residing under shared mount, then move_mount
will not work. This is documented here:

https://elixir.bootlin.com/linux/v6.17/source/Documentation/filesystems/sharedsubtree.rst#L497

"/" is shared by default if we booted using systemd. This is why
you observing EINVAL.

I just found that this is already documented in move_mount(2):

    EINVAL The  source  mount  object's  parent  mount  has  shared  mount propagation, and thus cannot be moved (as described in mount_name‐
    spaces(7)).

So everything is working as intended, and no changes to manual pages are
needed.

On the other hand, this is a good idea to add a bigger warning to
move_mount(2) (and to mount(2), it is affected, too). I. e. to add something
like this to main text of move_mount (as opposed to "ERRORS"):
"Note that systemd makes "/" shared by default. Moving mounts residing
under shared mounts is prohibited, so attempting to move attached
mount using move_mount likely will not work".

(I personally don't have plans to submit this as a patch.)

-- 
Askar Safin