Re: RFC: deprecating/removing the legacy mode of devpts

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

H. Peter Anvin wrote:
> Currently, devpts supports a legacy mode where only one instance exists.
>   For backwards compatibility, this is also the default mode, but the
> right thing should be to migrate everyone over to the "newinstance" scheme.
>
> However, it is increasingly clear that that is not happening; neither
> the distros nor udev support this by default at this point.
>
> I think it might be time we set a sunset date for the legacy mode,
> probably around kernel 3.9 at this point.
>
> The big change is that /dev/ptmx instead of being a device node should
> be a symlink to /dev/pts/ptmx, which is then either chmodded to the
> proper permissions, or given proper permissions via the ptmxmode mount
> options to devpts.  The latter is probably preferable from a technical
> point of view since it doesn't create a "gap".
>
> As such, I would like to suggest the following:
>
> 1. For 3.5, add this to feature-removal.txt.
> 2. For 3.6, remove CONFIG_DEVPTS_MULTIPLE_INSTANCES as a configuration
>     option, and printk a warning if devpts is mounted without the
>     "newinstance" option.
> 3. For 3.9 remove the legacy support including support for a ptmx node
>     outside the devpts filesystem.
>
> 	-hpa
>

I think we should give the way to test compatibility as early as possible, so:
2'. For 3.6 add CONFIG_DEVPTS_LEGACY_INSTANCE =y by default, if it =n:
     always create new instance (ignore "newinstance") and remove "ptmx" from sysfs.