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

From: Oren Laadan
Date: Tue Feb 05 2008 - 21:05:23 EST



Serge E. Hallyn wrote:
Quoting Oren Laadan (orenl@xxxxxxxxxxxxxxx):
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.

LOL ... a bit of misunderstanding - let me put some order here:

my response what with respect to the new interface that Pierre
suggested, that is - to add a new IPC call to change an identifier
after it has been allocated (and assigned). This is necessary for the
restart because applications expect to see the same resource id's as
they had at the time of the checkpoint.

What you are referring to is the more recent part of the thread, where
the topic became how data should be saved - in other words, the format
of the checkpoint data. This is entirely orthogonal to my argument.

Now please re-read my email :)

That said, I'd advocate for something in between a raw dump and a pure
"parametric" representation of the data. Raw data tends to be, well,
too raw, which makes the task of reading data from older version by
newer kernels harder to maintain. On the other hand, it is impossible
to abstract everything into kernel-independent format.


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.

You will never be able to avoid that completely, simply because new
kernels will require saving more (or less) data per object, because
of new (or dropped) features.
The best solution in this sense is to provide a filter (hopefully
in user space, utility) that would convert a checkpoint image file
from the old format to a newer format.
And you keep a lot of compatibility code of the kernel, too.


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?

(context switch back to my original reply)

I prefer not to add a new interface to IPC that will provide a new
functionality that isn't needed, except for the checkpoint - because
there is a better alternative to do the same task; this alternative
is more suitable because (a) it can be applied incrementally, (b) it
provides a consistent method to pre-select identifiers of all syscalls,
(where is the current suggestion suggests one way for IPC and will
suggest other hacks for other resources).

(context switch back to the current reply)

I definitely welcome a cleanup of the (insanely multiplexedd) IPC
code. However I argue that the interface need not be extended.


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...

:) well, assuming the selection of resource IDs is done as I suggested,
we'll have the restart use it. If someone finds a good reason (other
than checkpoint/restart) to pre-select/modify an identifier, it will
be easy to _then_ add an interface. That (hypothetical) interface is
likely to come out more clever after X months using checkpoint/restart.


** 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.

Consider that we want to have an ultra-fast restart, so we let processes
restart in parallel (as much as possible) in the same container. Task A
wants to allocate IPC id 256, but the kernel allocates 32; before task A
manages to change it to 256 (with the new interface), task B attempts to
create an IPC id 32; the kernel provides, say, 1024, and task B fails to
change it to 32 because it is still used by task A. So restart fails :(

On the other hand, if a process first tells the kernel "I want 32" and
then calls, for instance, semget(), then the IPC can atomically ensure
that the process gets what it wanted.


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?

I wasn't. That was Pavel. While I think the idea is neat, I'm not
convinced that it's practical and best way to go, however I need to
further think about it.

And as I said, I see this as a separate issue from the problem of
create_with_id()/set_id issue().


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.

Exactly ! With the /proc/self/... approach there will not be a need
for a clone_with_pid() extension in terms of user-visible interface;
makes the clone-flags headache a bit more manageable :p

Ah... ok, long one, hopefully clarifies the confusion. That said, I
suggest that the debate regarding the format of the checkpoint data
shall proceed on a new thread, since IMHO it's orthogonal.

Oren.


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@xxxxxxxx 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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