[Devel] Re: Per-instance devpts
sukadev at us.ibm.com
sukadev at us.ibm.com
Sat Aug 2 22:08:00 PDT 2008
H. Peter Anvin [hpa at zytor.com] wrote:
> Since the issue of PTY namespaces came up (and was rejected) back in April,
> I have thought a little bit about changing ptys to be tied directly into a
> devpts instance. devpts would then be a "normal" filesystem, which can be
> mounted multiple times (or not at all). pty's would then become private to
> a devpts instance.
Sorry, I thought we were going with a complete device namespace - since that
would address other devices as well and would avoid the following user-space
issue.
I guess this issue came up in OLS recently and have been looking into this
again. I have some helper patches to explore multiple mounts of devpts
without namespace stuff and can send them out in a couple of days.
>
> This is what it would appear would have to change, and I'd like to get
> people's feeing for the user-space impact:
>
> 1. /dev/ptmx would have to change to a symlink, ptmx -> pts/ptmx.
IIRC, /dev/tty also needs a similar symlink.
> 2. Permissions on /dev/ptmx would not be persistent, and would have to
> be set via devpts mount options (unless they're 0666 root.tty, which
> would presumably be the default.)
> 3. The /proc/sys/kernel/pty limit would be global; a per-filesystem
> limit could be added on top or instead (presumably via a filesystem
> mount options.)
>
> I worry #1 would have substantial user-space impact, but I don't see a way
> around it, since there would be no obvious way to associate /dev/ptmx with
> a filesystem.
Sukadev
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