[Devel] Re: [PATCH 4/4] The control group itself

[Pavel Emelyanov]

Paul Menage wrote:
> On Thu, Feb 7, 2008 at 5:01 AM, Pavel Emelyanov <xemul at openvz.org> wrote:
>>  +
>>  +[cb] <major>:(<minor>|*) [r-][w-]
>>  + ^          ^               ^
>>  + |          |               |
>>  + |          |               +--- access rights (1)
>>  + |          |
>>  + |          +-- device major and minor numbers (2)
>>  + |
>>  + +-- device type (character / block)
>> ...
>>  +When reading from it, one may see something like
>>  +
>>  +       c 1:5 rw
>>  +       b 8:* r-
>>  +
> 
> In the interest of avoiding proliferating cgroup control file formats,
> I'm wondering if we can abstract out the general form of the data
> being presented here and maybe simplify it in such a way that we can
> hopefully reuse the format for other control files in the future?
> 
> For example, one way to represent this would be a map from device
> strings such c:1:5 to permission strings such as rw. Or maybe
> numerical device ids to numerical permission values.

You mean smth like <some_device_id><space><some_permissions_string>?

Well, I don't mind, but AFAIK the <major>:<minor> form is very common
for specifying the device. So I agree with the 'c:1:5 rw' form.

> The alternative might be to accept that there are two kinds of control
> files - those which are likely to be programmatically read (e.g.
> resource usage values), and those that are likely to be
> programmatically written but only actually read by humans for
> debugging purposes (like this one) and make it clear up-front when a
> control file is added which type they're considered to be. We could
> then ignore the API consistency requirements for the
> debugging-readable files.

Hmm, you mean make them a binary files? I thought that filesystem-based
API should be human readable and writable as much as possible...

> On a separate note, can you document the recommended way to completely
> overwrite the set of device permissions for a cgroup? Does this

There's not way to flush all the permissions in this implementation, but 
I though about one. Maybe 'echo 0 > devices.permissions' would be good?

> involves writing a "--" permission for every device that you don't
> want in the cgroup?

Currently - yes.

> Paul
> 

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