Re: [PATCH 1/2] cgroup allow subsys to set default mode of its own file

[Paul Menage <menage@google.com>]

On Thu, Feb 26, 2009 at 1:00 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Wed, 25 Feb 2009 16:35:55 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>
>>  When I wrote tools for maintain cgroup, I can't find which file is
>>  writable intarfece or not via cgroup file systems. (finally, I did
>>  dirty approach.)
>>  IMHO, showing "this file is read-only" in explicit way is useful
>>  for user-land (tools). In other story, a file whose name sounds read-only
>>  may have "trigger" operation and support reseting. In this case,
>>  "writable" is informative.
>
> Well, we have compatibility issues here.  If we make this change, and
> people write tools which depend upon that change then those tools might
> break when run upon older kernels.

I don't think that's too big a deal - a write can always fail at the
whim of a cgroups subsystem, so this would just be a hint to a tool
that it shouldn't even bother trying to write to the file. It should
be able to handle a failure.

But I don't see why we can't figure out the mode automatically based
on whether or not there's a write handler defined for the control
file.

Paul