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

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Date:   Wed, 25 Aug 2021 09:38:02 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Christian König <christian.koenig@....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

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..

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.

Jason