Message-Id: <20080229093013.1e136398.pj@sgi.com>
Date:	Fri, 29 Feb 2008 09:30:13 -0600
From:	Paul Jackson <pj@....com>
To:	"Paul Menage" <menage@...gle.com>
Cc:	akpm@...ux-foundation.org, containers@...ts.osdl.org,
	linux-kernel@...r.kernel.org, balbir@...ux.vnet.ibm.com,
	a.p.zijlstra@...llo.nl, xemul@...nvz.org
Subject: Re: [RFC] [PATCH] Re: Prefixing cgroup generic control filenames
 with "cgroup."

Paul M wrote:
> Well, the additional components are called "groups" not "cgroups", but
> basically yes.

Ah yes - "groups" - right, sorry:

  /mnt/cgroup/groups/user_created_groupname1/groups/user_created_groupname2


> Yes, we could just say "the kernel reserves the right to use any names
> that begin with a lower-case letter, and no others", and be done with
> it.

The pattern might be stronger (more restrictive) than "[a-z].*"

The pattern might be something like:
 1) the known set of existing names (a short, specific list), plus
 2) "[a-z]+\.[a-z]+(_[a-z]+)*"

That second pattern is some lower case letters, a dot, and some
more lower case letters, possibly with some embedded underscores.

That is, except for the grandfathered existing known names, such as
"tasks", you would be promising that all names added in the future
would look like the examples (for some string of lower case letters
"subsys"):

	subsys.foo
	subsys.bar_baz

or for cgroup infrastructure names (using "kern" or "groups" prefix,
I don't have a clear preference):

	groups.stuff
	groups.this_and_that

Then for instance any name (not already in use) that had either (1) no
embedded dot '.', or (2) at least one character other than "[a-z_.]+",
or (3) other variants too numerous to list, would be safe for user
created group names.

Or, for a simpler and more restrictive regex pattern, don't allow
underscores, resulting in all kernel generated names matching:

 1) the known list, or
 2) "[a-z]+\.[a-z]+"

Note here "safe for user created" names just means safe from collision
with kernel generated names, not necessarily safe from collision with
other user generated names.

That is regardless of what you do here, you cannot protect the
delicate user from possible collision.  You can only protect them
from collision with "your" names.

This risks imposing extra complexity on users just so you can avoid
being blamed for the name collisions they might still experience
anyway.  When I phrase it that way, my enthusiasm for this proposal
weakens further.


> >  And did I say incompatible with released versions?
> 
> Not at all incompatible if it requires a mount option to enable it ...

Ah - are you saying I missed another detail?  That depending on how they
mounted it, the path might be either of:

  /mnt/cgroup/groups/user_created_groupname1/groups/user_created_groupname2
or
  /mnt/cgroup/user_created_groupname1/user_created_groupname2

So code that knows something about these paths has to work with either
form (since not all code using these paths will control the mount
relevant option.)

I hope I misunderstood something here.


> That leaves a bit of an ugly taste in my mouth  ...

Could you spell out the key reason -you- find it distasteful,  perhaps
for a stronger pattern such as "[a-z]+\.[a-z]+" I consider above?
Perhaps I'm missing some reason to share in your revulsion.


>  but if people seem to prefer that approach we can go for it.

So long as /dev/cpuset is unscathed, I'm ok either way.  Let's
see what others think.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@....com> 1.940.382.4214
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