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

[Leon Romanovsky <leon@kernel.org>]

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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@ziepe.ca> 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@tencent.com>
> >> > > > >  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.

Thanks

>
> >
> > Jason

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