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

[Jason Gunthorpe]

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.

Jason