linux-kernel - Re: [Linaro-mm-sig] Changing vma->vm_file in dma_buf

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Message-ID: <20200917163907.GI8409@ziepe.ca>
Date:   Thu, 17 Sep 2020 13:39:07 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Christian König <christian.koenig@....com>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>, Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] Changing vma->vm_file in dma_buf_mmap()

On Thu, Sep 17, 2020 at 06:06:14PM +0200, Christian König wrote:
> > > If it is already taking a page fault I'm not sure the extra function
> > > call indirection is going to be a big deal. Having a uniform VMA
> > > sounds saner than every driver custom rolling something.
> > > 
> > > When I unwound a similar mess in RDMA all the custom VMA stuff in the
> > > drivers turned out to be generally buggy, at least.
> > > 
> > > Is vma->vm_file->private_data universally a dma_buf pointer at least?
> > Nope. I think if you want this without some large scale rewrite of a
> > lot of code we'd need a vmops->get_dmabuf or similar. Not pretty, but
> > would get the job done.
> 
> Yeah, agree that sounds like the simplest approach.

I don't think that will fly, it is clearly only papering over a mess
inside DRM/dma buf :\

Jason