linux-kernel - Re: [RFC] Make use of non-dynamic dmabuf in RDMA

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Date:   Wed, 25 Aug 2021 15:51:14 +0200
From:   Christian König <christian.koenig@....com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Gal Pressman <galpress@...zon.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Doug Ledford <dledford@...hat.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Oded Gabbay <ogabbay@...ana.ai>,
        Tomer Tayar <ttayar@...ana.ai>,
        Yossi Leybovich <sleybo@...zon.com>,
        Alexander Matushevsky <matua@...zon.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Jianxin Xiong <jianxin.xiong@...el.com>
Subject: Re: [RFC] Make use of non-dynamic dmabuf in RDMA

Am 25.08.21 um 14:38 schrieb Jason Gunthorpe:
> On Wed, Aug 25, 2021 at 02:27:08PM +0200, Christian König wrote:
>> Am 25.08.21 um 14:18 schrieb Jason Gunthorpe:
>>> On Wed, Aug 25, 2021 at 08:17:51AM +0200, Christian König wrote:
>>>
>>>> The only real option where you could do P2P with buffer pinning are those
>>>> compute boards where we know that everything is always accessible to
>>>> everybody and we will never need to migrate anything. But even then you want
>>>> some mechanism like cgroups to take care of limiting this. Otherwise any
>>>> runaway process can bring down your whole system.
>>> Why? It is not the pin that is the problem, it was allocating GPU
>>> dedicated memory in the first place. pinning it just changes the
>>> sequence to free it. No different than CPU memory.
>> Pinning makes the memory un-evictable.
>>
>> In other words as long as we don't pin anything we can support as many
>> processes as we want until we run out of swap space. Swapping sucks badly
>> because your applications become pretty much unuseable, but you can easily
>> recover from it by killing some process.
>>
>> With pinning on the other hand somebody sooner or later receives an -ENOMEM
>> or -ENOSPC and there is no guarantee that this goes to the right process.
> It is not really different - you have the same failure mode once the
> system runs out of swap.
>
> This is really the kernel side trying to push a policy to the user
> side that the user side doesn't want..

But which is still the right thing to do as far as I can see. See 
userspace also doesn't want proper process isolation since it takes 
extra time.

Kernel development is driven by exposing the hardware functionality in a 
save and manageable manner to userspace, and not by fulfilling userspace 
requirements.

This is very important cause you otherwise you create a specialized 
system and not a general purpose kernel.

> Dedicated systems are a significant use case here and should be
> supported, even if the same solution wouldn't be applicable to someone
> running a desktop.

And exactly that approach is not acceptable.

Christian.

>
> Jason