linux-kernel - Re: Changing vma->vm_file in dma_buf

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Message-ID: <20200916140710.GA8409@ziepe.ca>
Date:   Wed, 16 Sep 2020 11:07:10 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     christian.koenig@....com, akpm@...ux-foundation.org,
        sumit.semwal@...aro.org, linux-media@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Changing vma->vm_file in dma_buf_mmap()

On Wed, Sep 16, 2020 at 11:53:59AM +0200, Daniel Vetter wrote:

> But within the driver, we generally need thousands of these, and that
> tends to bring fd exhaustion problems with it. That's why all the private
> buffer objects which aren't shared with other process or other drivers are
> handles only valid for a specific fd instance of the drm chardev (each
> open gets their own namespace), and only for ioctls done on that chardev.
> And for mmap we assign fake (but unique across all open fd on it) offsets
> within the overall chardev. Hence all the pgoff mangling and re-mangling.

Are they still unique struct files? Just without a fdno?
 
> Hence why we'd like to be able to forward aliasing mappings and adjust the
> file and pgoff, while hopefully everything keeps working. I thought this
> would work, but Christian noticed it doesn't really.

It seems reasonable to me that the dma buf should be the owner of the
VMA, otherwise like you say, there is a big mess attaching the custom
vma ops and what not to the proper dma buf.

I don't see anything obviously against this in mmap_region() - why did
Chritian notice it doesn't really work?

Jason