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

[858585 jemmy]

On Sat, May 5, 2018 at 2:23 AM, Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> On Fri, May 04, 2018 at 04:51:15PM +0800, 858585 jemmy wrote:
>> On Fri, May 4, 2018 at 11:14 AM, 858585 jemmy <jemmy858585@xxxxxxxxx> wrote:
>> > On Thu, May 3, 2018 at 11:33 PM, Jason Gunthorpe <jgg@xxxxxxxx> 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?
>> >
>> > commit 87773dd56d5405ac28119fcfadacefd35877c18f add pid in ib_umem structure.
>> >
>> > and the comment has such information:
>> > Later a different process with a different mm_struct than the one that
>> > allocated the ib_umem struct
>> > ends up releasing it which results in decrementing the new processes
>> > mm->pinned_vm count past
>> > zero and wrapping.
>>
>> I think a different process should not have the permission to release ib_umem.
>> so maybe the reason is not a different process?
>> can ib_umem_release be invoked in interrupt context?
>
> We plan to restore fork support and add some way to share MRs between
> processes, so we must consider having a different process release the
> umem than acquired it.

If restore fork support, what is the expected behavior?
If parent process pinned_vm is x, what is the child process pinned_vm
value after fork? It reset to zero now.
If the parent process call ibv_dereg_mr after fork, should the child
process decrease pinned_vm?
If the child process call  ibv_dereg_mr after fork, should the parent
process decrease pinned_vm?


>
> Jason