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

[Qi Zheng <zhengqi.arch@bytedance.com>]

Hi Steven,

The email you replied to was the failed version (due to the error
below), so I copied your reply and replied to you on this successful
version.

(4.7.1 Error: too many recipients from 49.7.199.173)

On 2023/6/23 18:01, Steven Price wrote:
 > 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.

Indeed. I will fix it in the v2.

Thanks,
Qi

 >
 > 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;
 >   }
 >
 >   /**
 >