[Devel] Re: [PATCH 2.6.24-rc8-mm1 09/15] (RFC) IPC: new kernel API to change an ID

[Serge E. Hallyn]

Quoting Oren Laadan (orenl at cs.columbia.edu):
>
> I strongly second Kirill on this matter.
>
> IMHO, we should _avoid_ as much as possible exposing internal kernel
> state to applications, unless a _real_ need for it is _clearly_
> demonstrated. The reasons for this are quite obvious.

Hmm, sure, but this sentence is designed to make us want to agree.  Yes,
we want to avoid exporting kernel internals, but generally that means
things like the precise layout of the task_struct.  What Pierre is doing
is in fact the opposite, exporting resource information in a kernel
version invariant way.

In fact, the very reason not to go the route you and Pavel are
advocating is that if we just dump task state to a file or filesystem
from the kernel in one shot, we'll be much more tempted to lay out data
in a way that exports and ends up depending on kernel internals.  So
we'll just want to read and write the task_struct verbatim.

So, there are two very different approaches we can start with.
Whichever one we follow, we want to avoid having kernel version
dependencies.  They both have their merits to be sure.

But note that in either case we need to deal with a bunch of locking.
So getting back to Pierre's patchset, IIRC 1-8 are cleanups worth
doing no matter 1.  9-11 sound like they are contentuous until
we decide whether we want to go with a create_with_id() type approach
or a set_id().  12 is IMO a good locking cleanup regardless.  13 and
15 are contentous until we decide whether we want userspace-controlled
checkpoint or a one-shot fs.  14 IMO is useful for both c/r approaches.

Is that pretty accurate?

> It isn't strictly necessary to export a new interface in order to
> support checkpoint/restart. **. Hence, I think that the speculation
> "we may need it in the future" is too abstract and isn't a good
> excuse to commit to a new, currently unneeded, interface.

OTOH it did succeed in starting some conversation :)

> Should the
> need arise in the future, it will be easy to design a new interface
> (also based on aggregated experience until then).

What aggregated experience?  We have to start somewhere...

> ** In fact, the suggested interface may prove problematic (as noted
> earlier in this thread): if you first create the resource with some
> arbitrary identifier and then modify the identifier (in our case,
> IPC id), then the restart procedure is bound to execute sequentially,
> because of lack of atomicity.

Hmm?  Lack of atomicity wrt what?  All the tasks being restarted were
checkpointed at the same time so there will be no conflict in the
requested IDs, so I don't know what you're referring to.

> That said, I suggest the following method instead (this is the method
> we use in Zap to determine the desired resource identifier when a new
> resource is allocated; I recall that we had discussed it in the past,
> perhaps the mini-summit in september ?):
>
> 1) The process/thread tells the kernel that it wishes to pre-determine
> the resource identifier of a subsequent call (this can be done via a
> new syscall, or by writing to /proc/self/...).
>
> 2) Each system call that allocates a resource and assigns an identifier
> is modified to check this per-thread field first; if it is set then
> it will attempt to allocate that particular value (if already taken,
> return an error, eg. EBUSY). Otherwise it will proceed as it is today.

But I thought you were just advocating a one-shot filesystem approach
for c/r, so we wouldn't be creating the resources piecemeal?

The /proc/self approach is one way to go, it has been working for LSMs
this long.  I'd agree that it would be nice if we could have a
consistent interface to the create_with_id()/set_id() problem.  A first
shot addressing ipcs and pids would be a great start.

> (I left out some details - eg. the kernel will keep the desire value
> on a per-thread field, when it will be reset, whether we want to also
> tag the field with its type and so on, but the idea is now clear).
>
> The main two advantages are that first, we don't need to devise a new
> method for every syscall that allocates said resources (sigh... just

Agreed.

> think of clone() nightmare to add a new argument);

Yes, and then there will need to be the clone_with_pid() extension on
top of that.

> second, the change
> is incremental: first code the mechanism to set the field, then add
> support in the IPC subsystem, later in the DEVPTS, then in clone and
> so forth.
>
> Oren.
>
> Pierre Peiffer wrote:
>> Kirill Korotaev wrote:
>>> Why user space can need this API? for checkpointing only?
>> I would say "at least for checkpointing"... ;) May be someone else may 
>> find an
>> interest about this for something else.
>> In fact, I'm sure that you have some interest in checkpointing; and thus, 
>> you
>> have probably some ideas in mind; but whatever the solution you will 
>> propose,
>> I'm pretty sure that I could say the same thing for your solution.
>> And what I finally think is: even if it's for "checkpointing only", if 
>> many
>> people are interested by this, it may be sufficient to push this ?
>>> Then I would not consider it for inclusion until it is clear how to 
>>> implement checkpointing.
>>> As for me personally - I'm against exporting such APIs, since they are 
>>> not needed in real-life user space applications and maintaining it 
>>> forever for compatibility doesn't worth it.
>> Maintaining these patches is not a big deal, really, but this is not the 
>> main
>> point; the "need in real life" (1) is in fact the main one, and then, the 
>> "is
>> this solution the best one ?" (2) the second one.
>> About (1), as said in my first mail, as the namespaces and containers are 
>> being
>> integrated into the mainline kernel, checkpoint/restart is (or will be) 
>> the next
>> need.
>> About (2), my solution propose to do that, as much as possible from 
>> userspace,
>> to minimize the kernel impact. Of course, this is subject to discussion. 
>> My
>> opinion is that doing a full checkpoint/restart from kernel space will 
>> need lot
>> of new specific and intrusive code; I'm not sure that this will be 
>> acceptable by
>> the community. But this is my opinion only. Discusion is opened.
>>> Also such APIs allow creation of non-GPL checkpointing in user-space, 
>>> which can be of concern as well.
>> Honestly, I don't think this really a concern at all. I mean: I've never 
>> seen
>> "this allows non-GPL binary and thus, this is bad" as an argument to 
>> reject a
>> functionality, but I may be wrong, and thus, it can be discussed as well.
>> I think the points (1) and (2) as stated above are the key ones.
>> Pierre
>>> Kirill
>>>
>>>
>>> Pierre Peiffer wrote:
>>>> Hi again,
>>>>
>>>> 	Thinking more about this, I think I must clarify why I choose this way.
>>>> In fact, the idea of these patches is to provide the missing user APIs 
>>>> (or
>>>> extend the existing ones) that allow to set or update _all_ properties 
>>>> of all
>>>> IPCs, as needed in the case of the checkpoint/restart of an application 
>>>> (the
>>>> current user API does not allow to specify an ID for a created IPC, for
>>>> example). And this, without changing the existing API of course.
>>>>
>>>> 	And msgget(), semget() and shmget() does not have any parameter we can 
>>>> use to
>>>> specify an ID.
>>>> 	That's why I've decided to not change these routines and add a new 
>>>> control
>>>> command, IP_SETID, with which we can can change the ID of an IPC. (that 
>>>> looks to
>>>> me more straightforward and logical)
>>>>
>>>> 	Now, this patch is, in fact, only a preparation for the patch 10/15 
>>>> which
>>>> really complete the user API by adding this IPC_SETID command.
>>>>
>>>> (... continuing below ...)
>>>>
>>>> Alexey Dobriyan wrote:
>>>>> On Tue, Jan 29, 2008 at 05:02:38PM +0100, pierre.peiffer at bull.net 
>>>>> wrote:
>>>>>> This patch provides three new API to change the ID of an existing
>>>>>> System V IPCs.
>>>>>>
>>>>>> These APIs are:
>>>>>> 	long msg_chid(struct ipc_namespace *ns, int id, int newid);
>>>>>> 	long sem_chid(struct ipc_namespace *ns, int id, int newid);
>>>>>> 	long shm_chid(struct ipc_namespace *ns, int id, int newid);
>>>>>>
>>>>>> They return 0 or an error code in case of failure.
>>>>>>
>>>>>> They may be useful for setting a specific ID for an IPC when preparing
>>>>>> a restart operation.
>>>>>>
>>>>>> To be successful, the following rules must be respected:
>>>>>> - the IPC exists (of course...)
>>>>>> - the new ID must satisfy the ID computation rule.
>>>>>> - the entry in the idr corresponding to the new ID must be free.
>>>>>>  ipc/util.c          |   48 
>>>>>> ++++++++++++++++++++++++++++++++++++++++++++++++
>>>>>>  ipc/util.h          |    1 +
>>>>>>  8 files changed, 197 insertions(+)
>>>>> For the record, OpenVZ uses "create with predefined ID" method which
>>>>> leads to less code. For example, change at the end is all we want from
>>>>> ipc/util.c .
>>>> And in fact, you do that from kernel space, you don't have the 
>>>> constraint to fit
>>>> the existing user API.
>>>> Again, this patch, even if it presents a new kernel API, is in fact a
>>>> preparation for the next patch which introduces a new user API.
>>>>
>>>> Do you think that this could fit your need ?
>>>>
>>>
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