[Devel] Re: [PATCH v6 02/10] ipc: "use key as id" functionality for resource get system call introduced

[Stanislav Kinsbursky]

15.10.2012 23:47, Eric W. Biederman пишет:
> ebiederm at xmission.com (Eric W. Biederman) writes:
>
>> Stanislav Kinsbursky <skinsbursky at parallels.com> writes:
>>
>>> This patch introduces new IPC resource get request flag IPC_PRESET, which
>>> should be interpreted as a request to try to allocate IPC slot with number,
>>> starting from value resented by key. IOW, kernel will try
>>> allocate new segment in specified slot.
>>>
>>> Note: if desired slot is not emply, then next free slot will be used.
>>
>> This way of handling things is pretty nasty.
>>
>> - You don't fail if the requested id is not available.
>> - You don't allow assigning the key (which leads to the need to change
>>    the key in later patches).  Changing the creator uid and creator
>>    gid and key is semantically ugly.
>>
>> It would be much cleaner if you could instead add IPC_PRESET and then
>> extend the definition of the creation functions all by one argument.
>>
>> aka
>> int msgget(key_t key, int msgflg, int id);
>> int semget(key_t key, int nsems, int semflg, int id);
>> int shmget(key_t key, size_t size, int shmflg, int id);
>>
>> Where the extra id argument is ignored unless IPC_PRESET is specified.
>>
>> Also msgget, semget, and shmget should fail if unregconized flags are
>> passed in.  That ipcget doesn't do that today is bizarre.
>
> Hmm.  Come to think of it I don't see why you need to set the id at all.
> We are using an idr allocator which effectively offers the semantics
> that the lowest available id will be allocated.  The same semantics we
> have for file descriptors.
>
> So it should be possible at least for the first pass at
> checkpoint/restart to implement the restoration of sysv ipc without
> IPC_PRESET at all.
>
> So IPC_PRESET should just be an optimization, not a necessary feature.
>

CRIU was designed to suspend/restore not only containers with it's own IPC 
namespace, but also for single process and process tree.
So we have to restore IPC objects with proper id and key.

> That makes all of your code go away except the message queue
> peeking, which seems much less intrusive for the first pass.
>
> Eric
>


-- 
Best regards,
Stanislav Kinsbursky