linux-kernel - Re: [PATCH] drm/prime: Ensure mmap offset is initialized

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Message-ID: <CAF6AEGuLeLmD4m+yi5csGdb0XZbnAOfYOKx6c-wEgMGt6rj7Cw@mail.gmail.com>
Date:   Mon, 30 May 2022 08:41:09 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        Rob Clark <robdclark@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Gerd Hoffmann <kraxel@...hat.com>,
        freedreno <freedreno@...ts.freedesktop.org>
Subject: Re: [PATCH] drm/prime: Ensure mmap offset is initialized

On Mon, May 30, 2022 at 7:49 AM Daniel Vetter <daniel@...ll.ch> wrote:
>
> On Mon, 30 May 2022 at 15:54, Rob Clark <robdclark@...il.com> wrote:
> >
> > On Mon, May 30, 2022 at 12:26 AM Thomas Zimmermann <tzimmermann@...e.de> wrote:
> > >
> > > Hi
> > >
> > > Am 29.05.22 um 18:29 schrieb Rob Clark:
> > > > From: Rob Clark <robdclark@...omium.org>
> > > >
> > > > If a GEM object is allocated, and then exported as a dma-buf fd which is
> > > > mmap'd before or without the GEM buffer being directly mmap'd, the
> > > > vma_node could be unitialized.  This leads to a situation where the CPU
> > > > mapping is not correctly torn down in drm_vma_node_unmap().
> > >
> > > Which drivers are affected by this problem?
> > >
> > > I checked several drivers and most appear to be initializing the offset
> > > during object construction, such as GEM SHMEM. [1] TTM-based drivers
> > > also seem unaffected. [2]
> > >
> > >  From a quick grep, only etnaviv, msm and omapdrm appear to be affected?
> > > They only seem to run drm_gem_create_mmap_offset() from their
> > > ioctl-handling code.
> > >
> > > If so, I'd say it's preferable to fix these drivers and put a
> > > drm_WARN_ONCE() into drm_gem_prime_mmap().
> >
> > That is good if fewer drivers are affected, however I disagree with
> > your proposal.  At least for freedreno userspace, a lot of bo's never
> > get mmap'd (either directly of via dmabuf), so we should not be
> > allocating a mmap offset unnecessarily.
>
> Does this actually matter in the grand scheme of things? We originally
> allocated mmap offset only on demand because userspace only had 32bit
> loff_t support and so simply couldn't mmap anything if the offset
> ended up above 32bit (even if there was still va space available).
>
> But those days are long gone (about 10 years or so) and the allocation
> overhead for an mmap offset is tiny. So I think unless you can
> benchmark an impact allocating it at bo alloc seems like the simplest
> design overall, and hence what we should be doing. And if the vma
> offset allocation every gets too slow due to fragmentation we can lift
> the hole tree from i915 into drm_mm and the job should be done. At
> that point we could also allocate the offset unconditionally in the
> gem_init function and be done with it.
>
> Iow I concur with Thomas here, unless there's hard data contrary
> simplicity imo trumps here.

32b userspace is still alive and well, at least on arm chromebooks ;-)

BR,
-R

> -Daniel
>
> >
> > BR,
> > -R
> >
> > > Best regards
> > > Thomas
> > >
> > > [1]
> > > https://elixir.bootlin.com/linux/v5.18/source/drivers/gpu/drm/drm_gem_shmem_helper.c#L85
> > > [2]
> > > https://elixir.bootlin.com/linux/v5.18/source/drivers/gpu/drm/ttm/ttm_bo.c#L1002
> > >
> > > >
> > > > Fixes: e5516553999f ("drm: call drm_gem_object_funcs.mmap with fake offset")
> > > > Signed-off-by: Rob Clark <robdclark@...omium.org>
> > > > ---
> > > > Note, it's possible the issue existed in some related form prior to the
> > > > commit tagged with Fixes.
> > > >
> > > >   drivers/gpu/drm/drm_prime.c | 5 +++++
> > > >   1 file changed, 5 insertions(+)
> > > >
> > > > diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> > > > index e3f09f18110c..849eea154dfc 100644
> > > > --- a/drivers/gpu/drm/drm_prime.c
> > > > +++ b/drivers/gpu/drm/drm_prime.c
> > > > @@ -716,6 +716,11 @@ int drm_gem_prime_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
> > > >       struct file *fil;
> > > >       int ret;
> > > >
> > > > +     /* Ensure that the vma_node is initialized: */
> > > > +     ret = drm_gem_create_mmap_offset(obj);
> > > > +     if (ret)
> > > > +             return ret;
> > > > +
> > > >       /* Add the fake offset */
> > > >       vma->vm_pgoff += drm_vma_node_start(&obj->vma_node);
> > > >
> > >
> > > --
> > > Thomas Zimmermann
> > > Graphics Driver Developer
> > > SUSE Software Solutions Germany GmbH
> > > Maxfeldstr. 5, 90409 Nürnberg, Germany
> > > (HRB 36809, AG Nürnberg)
> > > Geschäftsführer: Ivo Totev
>
>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch