Re: [PATCH 05/29] drm/panfrost: dynamically allocate the drm-panfrost shrinker

Steven Price
2023-06-23T10:01:29Z

On 22/06/2023 09:39, Qi Zheng wrote:
> From: Qi Zheng <zhengqi.arch@bytedance.com>
>
> In preparation for implementing lockless slab shrink,
> we need to dynamically allocate the drm-panfrost shrinker,
> so that it can be freed asynchronously using kfree_rcu().
> Then it doesn't need to wait for RCU read-side critical
> section when releasing the struct panfrost_device.
>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
> drivers/gpu/drm/panfrost/panfrost_device.h | 2 +-
> .../gpu/drm/panfrost/panfrost_gem_shrinker.c | 24 ++++++++++---------
> 2 files changed, 14 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/gpu/drm/panfrost/panfrost_device.h b/drivers/gpu/drm/panfrost/panfrost_device.h
> index b0126b9fbadc..e667e5689353 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_device.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_device.h
> @@ -118,7 +118,7 @@ struct panfrost_device {
>
> struct mutex shrinker_lock;
> struct list_head shrinker_list;
> - struct shrinker shrinker;
> + struct shrinker *shrinker;
>
> struct panfrost_devfreq pfdevfreq;
> };
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> index bf0170782f25..2a5513eb9e1f 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> @@ -18,8 +18,7 @@
> static unsigned long
> panfrost_gem_shrinker_count(struct shrinker *shrinker, struct shrink_control *sc)
> {
> - struct panfrost_device *pfdev =
> - container_of(shrinker, struct panfrost_device, shrinker);
> + struct panfrost_device *pfdev = shrinker->private_data;
> struct drm_gem_shmem_object *shmem;
> unsigned long count = 0;
>
> @@ -65,8 +64,7 @@ static bool panfrost_gem_purge(struct drm_gem_object *obj)
> static unsigned long
> panfrost_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
> {
> - struct panfrost_device *pfdev =
> - container_of(shrinker, struct panfrost_device, shrinker);
> + struct panfrost_device *pfdev = shrinker->private_data;
> struct drm_gem_shmem_object *shmem, *tmp;
> unsigned long freed = 0;
>
> @@ -100,10 +98,15 @@ panfrost_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
> void panfrost_gem_shrinker_init(struct drm_device *dev)
> {
> struct panfrost_device *pfdev = dev->dev_private;
> - pfdev->shrinker.count_objects = panfrost_gem_shrinker_count;
> - pfdev->shrinker.scan_objects = panfrost_gem_shrinker_scan;
> - pfdev->shrinker.seeks = DEFAULT_SEEKS;
> - WARN_ON(register_shrinker(&pfdev->shrinker, "drm-panfrost"));
> +
> + pfdev->shrinker = shrinker_alloc_and_init(panfrost_gem_shrinker_count,
> + panfrost_gem_shrinker_scan, 0,
> + DEFAULT_SEEKS, 0, pfdev);
> + if (pfdev->shrinker &&
> + register_shrinker(pfdev->shrinker, "drm-panfrost")) {
> + shrinker_free(pfdev->shrinker);
> + WARN_ON(1);
> + }

So we didn't have good error handling here before, but this is
significantly worse. Previously if register_shrinker() failed then the
driver could safely continue without a shrinker - it would waste memory
but still function.

However we now have two failure conditions:
* shrinker_alloc_init() returns NULL. No warning and NULL deferences
will happen later.

* register_shrinker() fails, shrinker_free() will free pdev->shrinker
we get a warning, but followed by a use-after-free later.

I think we need to modify panfrost_gem_shrinker_init() to be able to
return an error, so a change something like the below (untested) before
your change.

Steve

----8<---
diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c
b/drivers/gpu/drm/panfrost/panfrost_drv.c
index bbada731bbbd..f705bbdea360 100644
--- a/drivers/gpu/drm/panfrost/panfrost_drv.c
+++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
@@ -598,10 +598,14 @@ static int panfrost_probe(struct platform_device
*pdev)
if (err < 0)
goto err_out1;

- panfrost_gem_shrinker_init(ddev);
+ err = panfrost_gem_shrinker_init(ddev);
+ if (err)
+ goto err_out2;

return 0;

+err_out2:
+ drm_dev_unregister(ddev);
err_out1:
pm_runtime_disable(pfdev->dev);
panfrost_device_fini(pfdev);
diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.h
b/drivers/gpu/drm/panfrost/panfrost_gem.h
index ad2877eeeccd..863d2ec8d4f0 100644
--- a/drivers/gpu/drm/panfrost/panfrost_gem.h
+++ b/drivers/gpu/drm/panfrost/panfrost_gem.h
@@ -81,7 +81,7 @@ panfrost_gem_mapping_get(struct panfrost_gem_object *bo,
void panfrost_gem_mapping_put(struct panfrost_gem_mapping *mapping);
void panfrost_gem_teardown_mappings_locked(struct panfrost_gem_object *bo);

-void panfrost_gem_shrinker_init(struct drm_device *dev);
+int panfrost_gem_shrinker_init(struct drm_device *dev);
void panfrost_gem_shrinker_cleanup(struct drm_device *dev);

#endif /* __PANFROST_GEM_H__ */
diff --git a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
index bf0170782f25..90265b37636f 100644
--- a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
+++ b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
@@ -97,13 +97,17 @@ panfrost_gem_shrinker_scan(struct shrinker
*shrinker, struct shrink_control *sc)
*
* This function registers and sets up the panfrost shrinker.
*/
-void panfrost_gem_shrinker_init(struct drm_device *dev)
+int panfrost_gem_shrinker_init(struct drm_device *dev)
{
struct panfrost_device *pfdev = dev->dev_private;
+ int ret;
+
pfdev->shrinker.count_objects = panfrost_gem_shrinker_count;
pfdev->shrinker.scan_objects = panfrost_gem_shrinker_scan;
pfdev->shrinker.seeks = DEFAULT_SEEKS;
- WARN_ON(register_shrinker(&pfdev->shrinker, "drm-panfrost"));
+ ret = register_shrinker(&pfdev->shrinker, "drm-panfrost");
+
+ return ret;
}

/**