Re: [PATCH v8 1/5] RDMA/umem: Support importing dma-buf as user memory region

From: Daniel Vetter <daniel@ffwll.ch>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: "Xiong, Jianxin" <jianxin.xiong@intel.com>,
	Leon Romanovsky <leon@kernel.org>,
	"linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	Doug Ledford <dledford@redhat.com>,
	"Vetter, Daniel" <daniel.vetter@intel.com>,
	Christian Koenig <christian.koenig@amd.com>
Subject: Re: [PATCH v8 1/5] RDMA/umem: Support importing dma-buf as user memory region
Date: Tue, 10 Nov 2020 15:14:45 +0100	[thread overview]
Message-ID: <20201110141445.GI401619@phenom.ffwll.local> (raw)
In-Reply-To: <20201106163953.GR36674@ziepe.ca>

On Fri, Nov 06, 2020 at 12:39:53PM -0400, Jason Gunthorpe wrote:
> On Fri, Nov 06, 2020 at 04:34:07PM +0000, Xiong, Jianxin wrote:
> 
> > > The user could specify a length that is beyond the dma buf, can
> > > the dma buf length be checked during get?
> > 
> > In order to check the length, the buffer needs to be mapped. That can be done.
> 
> Do DMA bufs even have definitate immutable lengths? Going to be a
> probelm if they can shrink

Yup. Unfortunately that's not document in the structures themselves,
there's just some random comments sprinkled all over that dma-buf size is
invariant, e.g. see the @mmap kerneldoc in dma_buf_ops:

https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html?highlight=dma_buf_ops#c.dma_buf_ops

"Because dma-buf buffers have invariant size over their lifetime, ..."

Jianxin, can you pls do a kerneldoc patch which updates the comment for
dma_buf.size and dma_buf_export_info.size?

Thanks, Daniel

> 
> > > Also page_size can be 0 because iova is not OK. iova should be
> > > checked for alignment during get as well:
> > > 
> > >   iova & (PAGE_SIZE-1) == umem->addr & (PAGE_SIZE-1)
> > 
> > If ib_umem_dmabuf_map_pages is called during get this error is automatically caught.
> 
> The ib_umem_find_best_pgsz() checks this equation, yes.
> 
> So you'd map the sgl before allocating the mkey? This then checks the
> length and iova?
> 
> Jason
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch