Re: [RFC][PATCH 0/4] Object creation with a specified id

[Nadia Derbey <Nadia.Derbey-6ktuUTfB/bM@public.gmane.org>]

Oren Laadan wrote:
> 
> 
> Nadia.Derbey-6ktuUTfB/bM@public.gmane.org wrote:
> 
>> A couple of weeks ago, a discussion has started after Pierre's 
>> proposal for
>> a new syscall to change an ipc id (see thread
>> http://lkml.org/lkml/2008/1/29/209).
>>
>>
>> Oren's suggestion was to force an object's id during its creation, rather
>> than 1. create it, 2. change its id.
>>
>> So here is an implementation of what Oren has suggested.
>>
>> 2 new files are defined under /proc/self:
>>   . next_ipcid --> next id to use for ipc object creation
>>   . next_pids --> next upid nr(s) to use for next task to be forked
>>                   (see patch #2 for more details).
> 
> 
> Generally looks good. One meta-comment, though:
> 
> I wonder why you use separate files for separate resources, 

That would be needed in a situation wheere we don't care about next, 
say, ipc id to be created but we need a predefined pid. But I must admit 
I don't see any pratical application to it.

> and why you'd
> want to write multiple identifiers in one go;

I used multiple identifiers only for the pid values: this is because 
when a new pid value is allocated for a process that belongs to nested 
namespaces, the lower level upid nr values are allocated in a single 
shot. (see alloc_pid()).

> it seems to complicate the
> code and interface with minimal gain.
> In practice, a process will only do either one or the other, so a single
> file is enough (e.g. "next_id").
> Also, writing a single value at a time followed by the syscall is enough;
> it's definitely not a performance issue to have multiple calls.
> We assume the user/caller knows what she's doing, so no need to classify
> the identifier (that is, tell the kernel it's a pid, or an ipc id) ahead
> of time. The caller simply writes a value and then calls the relevant
> syscall, or otherwise the results may not be what she expected...
> If such context is expected to be required (although I don't see any at
> the moment),  we can require that the user write "TYPE VALUE" pair to
> the "next_id" file.

That's exactly what I wanted to avoid by creating 1 file per object.
Now, it's true that in a restart context where I guess that things will 
be done synchronously, we could have a single next_id file.

> 
>>
>> When one of these files (or both of them) is filled, a structure 
>> pointed to
>> by the calling task struct is filled with these ids.
>>
>> Then, when the object is created, the id(s) present in that structure are
>> used, instead of the default ones.
>>
>> The patches are against 2.6.25-rc3-mm1, in the following order:
>>
>> [PATCH 1/4] adds the procfs facility for next ipc to be created.
>> [PATCH 2/4] adds the procfs facility for next task to be forked.
>> [PATCH 3/4] makes use of the specified id (if any) to allocate the new 
>> IPC
>>             object (changes the ipc_addid() path).
>> [PATCH 4/4] uses the specified id(s) (if any) to set the upid nr(s) 
>> for a newly
>>             allocated process (changes the alloc_pid()/alloc_pidmap() 
>> paths).
>>
>> Any comment and/or suggestions are welcome.
>>
>> Cc-ing Pavel and Sukadev, since they are the pid namespace authors.
>>
>> Regards,
>> Nadia
>>
>> -- 
>>
>> -- 
> 
> 
> 


Regards,
Nadia