Hi Serge,
>> + /* Read-only bind-mounting... In older kernels, doing that
>> required
>> + * to do one MS_BIND mount and then MS_REMOUNT|MS_RDONLY the
>> same
>> + * one. According to mount(2) manpage, MS_BIND honors MS_RDONLY
>> from
>> + * kernel 2.6.26 onwards. However, this apparently does not
>> work
>> on
>> + * kernel 3.8.
>
> I'm still not sure what is going on for you with this. Running with
> the
> latest 3.11 userns-enabled kernel (at ppa:ubuntu-lxc/kernel), I can
> still do
>
> sudo mkdir /tmp/{a,b}
> sudo mount -t tmpfs tmpfs /tmp/a
> sudo mount --bind /tmp/a /tmp/b
> sudo mount -o remount,bind,ro /tmp/b
>
> and end up with /tmp/b roand /tmp/a rw. This works for me in 3.2,
> 3.8,
> and userns-enabled 3.11.
I meant with that comment that mount --bind -o ro /tmp/a /tmp/b doesn't
work directly, at least with 3.8, but you have to do two steps. This
does not appear to be in accordance with the mount(2) manpage.
>> Unfortunately, on that very same kernel, doing the
>> + * same trick as above doesn't seem to work either, there one
>> needs
>> + * to ALSO specify MS_BIND for the remount, otherwise the entire
>> + * fs is remounted read-only or the mount fails because it's
>> busy...
>
> That's actually understandable - there are two things which both have
> a readonly.vs.readwrite state - the vfsmount, and the superblock. If
> you don't add MS_BIND, then you are requesting remount of the
> superblock.
Yes, I see what you mean, but this is definitely a change in the
behaviour of the kernel compared to previous versions. And that
also means that (see other thread) bind-mounting a rootfs onto
itself will not prevent a container from remounting the filesystem
readonly on shutdown.
Nevertheless, this should not be a problem for THIS patch, the
current version should work with any reasonable kernel you may
want to use LXC with.
-- Christian
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