On Tue, Nov 29, 2022 at 12:02:42PM -0800, Rob Clark wrote:
> From: Rob Clark <robdcl...@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: sta...@vger.kernel.org
> Signed-off-by: Rob Clark <robdcl...@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);
> }
