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

[Guenter Roeck <linux@roeck-us.net>]

On Tue, Nov 29, 2022 at 12:02:42PM -0800, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.org>
> 
> 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.

> 
> Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
> Cc: stable@vger.kernel.org
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> ---
>  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);
>  }