[Toybox] Remount feature of mount

[Liwei Jiang]

My /proc/mounts is an empty file.

On Tue, Aug 1, 2017 at 4:12 PM, Rob Landley <rob at landley.net> wrote:

> On 07/31/2017 11:30 PM, Liwei Jiang wrote:
> > No. I have pulled and compiled the latest code. The same problem still
> > exist. It reports, "mount: '/usr/local' not in /proc/mounts"
>
> That's a separate error.
>
> Could you show me your /proc/mounts file?
>
> Remount needs to get the previous mount's flags from /proc/mounts. For
> example, my root filesystem (ubuntu 14.04's defaults) has:
>
>   $ grep sda1 /proc/mounts
>   /dev/sda1 / ext4 rw,relatime,errors=remount-ro,data=ordered 0 0
>
> Without that, remount doesn't (for example) know to set the relatime
> flag on the remount, and if it doesn't that flag gets removed. A remount
> has to apply your current flags as an offset to the previous flags,
> meaning it needs to know the previous flags.
>
> Way back in the dark ages mount used to maintain its own file
> (/etc/mtab) with this info, but this couldn't reliably keep track of the
> data: for example if you mounted or umounted anything in an overlapping
> chroot, the host's /etc/mtab wouldn't get the updates. If a program ever
> mounted anything with the system call instead of the mount command,
> /etc/mtab wouldn't be updated. When the 'shared subtree' stuff went into
> 2.6.15 mounts became a per-process attribute and a single systemwide
> /etc/mtab made no _sense_ anymore. And then we built containers on top
> of that combining the "in a chroot" with "per-process-attribute" and
> everybody finally gave up on /etc/mtab. (I was arguing they _should_ do
> so back in 2005, but nobody listened back then...)
>
> Since the kernel knows this information, the kernel exports it in
> mtab/fstab format, so people could symlink /etc/mtab to that and always
> get the right information. Newer mount commands just look for it
> straight out of proc without needing the symlink.
>
> Rob
>



-- 
Dragonslayer
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