Re: [PATCH] IB/umem: use tgid instead of pid in ib_umem structure

[lidongchen(陈立东)]

> 在 2018年5月4日，21:39，Leon Romanovsky <leon@xxxxxxxxxx> 写道：
> 
>> On Fri, May 04, 2018 at 04:32:38PM +0800, 858585 jemmy wrote:
>>> On Fri, May 4, 2018 at 6:01 AM, Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>>>> On Thu, May 03, 2018 at 09:43:01PM +0300, Leon Romanovsky wrote:
>>>>> On Thu, May 03, 2018 at 12:26:56PM -0600, Jason Gunthorpe wrote:
>>>>>> On Thu, May 03, 2018 at 09:12:35PM +0300, Leon Romanovsky wrote:
>>>>>>> On Thu, May 03, 2018 at 09:33:10AM -0600, Jason Gunthorpe wrote:
>>>>>>>> On Thu, May 03, 2018 at 10:04:34PM +0800, Lidong Chen wrote:
>>>>>>>> The userspace may invoke ibv_reg_mr and ibv_dereg_mr by different threads.
>>>>>>>> If when ibv_dereg_mr invoke and the thread which invoked ibv_reg_mr has
>>>>>>>> exited, get_pid_task will return NULL, ib_umem_release does not decrease
>>>>>>>> mm->pinned_vm. This patch fixes it by use tgid.
>>>>>>>> 
>>>>>>>> Signed-off-by: Lidong Chen <lidongchen@xxxxxxxxxxx>
>>>>>>>> drivers/infiniband/core/umem.c | 12 ++++++------
>>>>>>>> include/rdma/ib_umem.h         |  2 +-
>>>>>>>> 2 files changed, 7 insertions(+), 7 deletions(-)
>>>>>>> 
>>>>>>> Why are we even using a struct pid for this? Does anyone know?
>>>>>>> 
>>>>>> 
>>>>>> Can it be related to "fork" support?
>>>>> 
>>>>> Not sure..
>>>>> 
>>>>> Ideally we want to hold the struct mm, but we can't hold it long
>>>>> term, so pid is a surrogate for that.
>>>>> 
>>>>>>> I'm surprised that struct task isn't held in the struct ib_umem..
>>>>>>> 
>>>>>> 
>>>>>> I think that this code can be removed and all accesses to mm_struct can
>>>>>> be done with "current->mm".
>>>>> 
>>>>> That sounds wrong for fork support, as the mm used in destroy MUST
>>>>> exactly match the mm used in create..
>>>>> 
>>>>> How does this accounting work in fork anyhow?
>>>> 
>>>> We are not supporting fork, so this is why I proposed to remove it.
>>> 
>>> Er, the new kabi certainly can call reg and dereg across a fork
>> 
>> what is the expect behavior after fork?
>> I write a test code, the dereg just return EACCES in the child
>> process. and have no effect.
> 
> Did you do reg/dereg over write() interface? If yes, this is expected
> behaviour of "not-supported fork()". A couple of months/years ago, your
> test program would work, but we closed this option due to security
> constraints.

the parent process call ibv_reg_mr, and the child process call ibv_dereg_mr.

If fork is not supported now, so use tgid to get mm structure is fine for multithread.

> 
> Thanks
> 
>> 
>>> 
>>> Jason