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

Kirill Korotaev dev at sw.ru
Thu Jan 31 01:54:10 PST 2008

Why user space can need this API? for checkpointing only?
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.
Also such APIs allow creation of non-GPL checkpointing in user-space, which can be of concern as well.


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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