linux-kernel - Re: [PATCH v12 1/5] dma-buf: Add dma-buf heaps framework

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Date:   Mon, 21 Oct 2019 09:35:47 +0000
From:   Brian Starkey <Brian.Starkey@....com>
To:     John Stultz <john.stultz@...aro.org>
CC:     lkml <linux-kernel@...r.kernel.org>,
        "Andrew F. Davis" <afd@...com>, Laura Abbott <labbott@...hat.com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Pratik Patel <pratikp@...eaurora.org>,
        Vincent Donnefort <Vincent.Donnefort@....com>,
        Sudipto Paul <Sudipto.Paul@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Chenbo Feng <fengc@...gle.com>,
        Alistair Strachan <astrachan@...gle.com>,
        Hridya Valsaraju <hridya@...gle.com>,
        Hillf Danton <hdanton@...a.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        nd <nd@....com>
Subject: Re: [PATCH v12 1/5] dma-buf: Add dma-buf heaps framework

On Fri, Oct 18, 2019 at 11:26:52AM -0700, John Stultz wrote:
> On Fri, Oct 18, 2019 at 4:18 AM Brian Starkey <Brian.Starkey@....com> wrote:
> > On Fri, Oct 18, 2019 at 05:23:19AM +0000, John Stultz wrote:
> >
> > As in v3:
> >
> >  * Avoid EXPORT_SYMBOL until we finalize modules (suggested by
> >    Brian)
> 
> Heh. I guess it has been awhile.  :)
> 
> > Did something change in that regard? I still think letting modules
> > register heaps without a way to remove them is a recipe for issues.
> 
> So yea, in recent months, work around Android with their GKI effort
> has made it necessary for ION heaps to be loadable from modules. I had
> some patches in WIP tree to enable this, and in the rework I did
> yesterday for the CMA module trivially collided with parts, and
> forgetting the discussion back in v3, I figured I'd just fold those
> bits in before I resubmitted for v12.

Ah yes, I can see that would be needed.

> 
> If it's an issue, I can pull it out, but I'm going to be submitting
> module enablement for review as soon as the core bits are queued, as
> its going to be important to support for Android to switch to this
> from ION.
> 

I don't know how to decide if it's an issue. My understanding is that
if you rmmod something which has exported buffers, various Bad Things
might happen; I believe including data leaks, corruption or crashing
the kernel. There's probably plenty of scope for that with dma-buf
exporters already, so it's not exactly "new" but it is a bit
unfortunate.

If "people" are OK with adding new code which has the same issue, then
I'm not going to make any more of a fuss, because perfection is the
enemy of progress. Perhaps an obvious comment pointing out the issue
would be prudent, though - as a reminder to people that add heaps from
their code (and wonder why there's no "dma_heap_remove" function).

Cheers,
-Brian

> thanks
> -john