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Date:   Tue, 13 Jul 2021 11:30:50 +0300
From:   Leon Romanovsky <leon@kernel.org>
To:     Shunsuke Mie <mie@igel.co.jp>
Cc:     linux-rdma@vger.kernel.org, Takanari Hayama <taki@igel.co.jp>
Subject: Re: [RFC] RDMA with Continuous Memory Allocator
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On Tue, Jul 13, 2021 at 03:57:53PM +0900, Shunsuke Mie wrote:
> Hi Leon,
> Thank you for your reply.
> 
> > Sorry for my question, but why do you need it?
> I'd like to write data to the buffer, prepared by DRM (gpu) driver as
> a frame buffer, using RDMA.
> There is a similar project as follows.
> https://www.openfabrics.org/wp-content/uploads/2020-workshop-presentations/303.-OFI-GPU-DMA-BUF-OFA2020v2.pdf
> They prepare a mechanism to share a dmabuf fd between DRM driver and
> RDMA driver, in order to update frame buffer using RDMA. I'm trying to
> develop that in userland.
> 
> Some DRM drivers use CMA to allocate the buffer. I met the problem in
> an environment that CMA used.

In some flows, dma-buf pins memory under the hood.

Anyway, for regular umem, you can try to use mlx5 that supports on
demand paging (ODP). That feature allows you to create umem without
need to pin pages.

Thanks