linux-kernel - Re: [PATCH] IB/umem: use tgid instead of pid in ib

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Date:   Fri, 4 May 2018 16:32:38 +0800
From:   858585 jemmy <jemmy858585@...il.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Leon Romanovsky <leon@...nel.org>, dledford@...hat.com,
        akpm@...ux-foundation.org, qing.huang@...cle.com,
        artemyko@...lanox.com, dan.j.williams@...el.com,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        adido@...lanox.com, Gal Shachaf <galsha@...lanox.com>,
        Aviad Yehezkel <aviadye@...lanox.com>,
        Lidong Chen <lidongchen@...cent.com>
Subject: Re: [PATCH] IB/umem: use tgid instead of pid in ib_umem structure

On Fri, May 4, 2018 at 6:01 AM, Jason Gunthorpe <jgg@...pe.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@...cent.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.

>
> Jason