Re: [2/2] drm/shmem-helper: Avoid vm_open error paths

[Rob Clark]

On Tue, Nov 29, 2022 at 12:32 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
>
> On Tue, Nov 29, 2022 at 12:02:42PM -0800, Rob Clark wrote:
> > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> >
> > vm_open() is not allowed to fail.  Fortunately we are guaranteed that
> > the pages are already pinned, and only need to increment the refcnt.  So
> > just increment it directly.
>
> I don't know anything about drm or gem, but I am wondering _how_
> this would be guaranteed. Would it be through the pin function ?
> Just wondering, because that function does not seem to be mandatory.

We've pinned the pages already in mmap.. vm->open() is perhaps not the
best name for the callback function, but it is called for copying an
existing vma into a new process (and for some other cases which do not
apply here because VM_DONTEXPAND).

(Other drivers pin pages in the fault handler, where there is actually
potential to return an error, but that change was a bit more like
re-writing shmem helper ;-))

BR,
-R

> >
> > Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > ---
> >  drivers/gpu/drm/drm_gem_shmem_helper.c | 14 +++++++++++---
> >  1 file changed, 11 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > index 110a9eac2af8..9885ba64127f 100644
> > --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> > +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > @@ -571,12 +571,20 @@ static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
> >  {
> >       struct drm_gem_object *obj = vma->vm_private_data;
> >       struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
> > -     int ret;
> >
> >       WARN_ON(shmem->base.import_attach);
> >
> > -     ret = drm_gem_shmem_get_pages(shmem);
> > -     WARN_ON_ONCE(ret != 0);
> > +     mutex_lock(&shmem->pages_lock);
> > +
> > +     /*
> > +      * We should have already pinned the pages, vm_open() just grabs
>
> should or guaranteed ? This sounds a bit weaker than the commit
> description.
>
> > +      * an additional reference for the new mm the vma is getting
> > +      * copied into.
> > +      */
> > +     WARN_ON_ONCE(!shmem->pages_use_count);
> > +
> > +     shmem->pages_use_count++;
> > +     mutex_unlock(&shmem->pages_lock);
>
> The previous code, in that situation, would not increment pages_use_count,
> and it would not set not set shmem->pages. Hopefully, it would not try to
> do anything with the pages it was unable to get. The new code assumes that
> shmem->pages is valid even if pages_use_count is 0, while at the same time
> taking into account that this can possibly happen (or the WARN_ON_ONCE
> would not be needed).
>
> Again, I don't know anything about gem and drm, but it seems to me that
> there might now be a severe problem later on if the WARN_ON_ONCE()
> ever triggers.
>
> Thanks,
> Guenter
>
> >
> >       drm_gem_vm_open(vma);
> >  }