Message-ID: <20251001132739.41575fa5@fedora>
Date: Wed, 1 Oct 2025 13:27:39 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Danilo Krummrich <dakr@...nel.org>, Matthew Brost
 <matthew.brost@...el.com>, "Thomas Hellström"
 <thomas.hellstrom@...ux.intel.com>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Steven Price <steven.price@....com>,
 Daniel Almeida <daniel.almeida@...labora.com>, Liviu Dudau
 <liviu.dudau@....com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v3 1/2] drm/gpuvm: add deferred vm_bo cleanup

On Wed, 01 Oct 2025 10:41:36 +0000
Alice Ryhl <aliceryhl@...gle.com> wrote:

> When using GPUVM in immediate mode, it is necessary to call
> drm_gpuvm_unlink() from the fence signalling critical path. However,
> unlink may call drm_gpuvm_bo_put(), which causes some challenges:
> 
> 1. drm_gpuvm_bo_put() often requires you to take resv locks, which you
>    can't do from the fence signalling critical path.
> 2. drm_gpuvm_bo_put() calls drm_gem_object_put(), which is often going
>    to be unsafe to call from the fence signalling critical path.
> 
> To solve these issues, add a deferred version of drm_gpuvm_unlink() that
> adds the vm_bo to a deferred cleanup list, and then clean it up later.
> 
> The new methods take the GEMs GPUVA lock internally rather than letting
> the caller do it because it also needs to perform an operation after
> releasing the mutex again. This is to prevent freeing the GEM while
> holding the mutex (more info as comments in the patch). This means that
> the new methods can only be used with DRM_GPUVM_IMMEDIATE_MODE.
> 
> Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  drivers/gpu/drm/drm_gpuvm.c | 184 ++++++++++++++++++++++++++++++++++++++++++++
>  include/drm/drm_gpuvm.h     |  16 ++++
>  2 files changed, 200 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_gpuvm.c b/drivers/gpu/drm/drm_gpuvm.c
> index a52e95555549a16c062168253477035679d4775b..a530cf0864c5dd837840f31d3e698d4a82c58d3c 100644
> --- a/drivers/gpu/drm/drm_gpuvm.c
> +++ b/drivers/gpu/drm/drm_gpuvm.c
> @@ -876,6 +876,27 @@ __drm_gpuvm_bo_list_add(struct drm_gpuvm *gpuvm, spinlock_t *lock,
>  	cond_spin_unlock(lock, !!lock);
>  }
>  
> +/**
> + * drm_gpuvm_bo_is_zombie() - check whether this vm_bo is scheduled for cleanup
> + * @vm_bo: the &drm_gpuvm_bo
> + *
> + * When a vm_bo is scheduled for cleanup using the bo_defer list, it is not
> + * immediately removed from the evict and extobj lists if they are protected by
> + * the resv lock, as we can't take that lock during run_job() in immediate
> + * mode. Therefore, anyone iterating these lists should skip entries that are
> + * being destroyed.
> + *
> + * Checking the refcount without incrementing it is okay as long as the lock
> + * protecting the evict/extobj list is held for as long as you are using the
> + * vm_bo, because even if the refcount hits zero while you are using it, freeing
> + * the vm_bo requires taking the list's lock.
> + */
> +static bool
> +drm_gpuvm_bo_is_zombie(struct drm_gpuvm_bo *vm_bo)
> +{
> +	return !kref_read(&vm_bo->kref);
> +}
> +
>  /**
>   * drm_gpuvm_bo_list_add() - insert a vm_bo into the given list
>   * @__vm_bo: the &drm_gpuvm_bo
> @@ -1081,6 +1102,8 @@ drm_gpuvm_init(struct drm_gpuvm *gpuvm, const char *name,
>  	INIT_LIST_HEAD(&gpuvm->evict.list);
>  	spin_lock_init(&gpuvm->evict.lock);
>  
> +	init_llist_head(&gpuvm->bo_defer);
> +
>  	kref_init(&gpuvm->kref);
>  
>  	gpuvm->name = name ? name : "unknown";
> @@ -1122,6 +1145,8 @@ drm_gpuvm_fini(struct drm_gpuvm *gpuvm)
>  		 "Extobj list should be empty.\n");
>  	drm_WARN(gpuvm->drm, !list_empty(&gpuvm->evict.list),
>  		 "Evict list should be empty.\n");
> +	drm_WARN(gpuvm->drm, !llist_empty(&gpuvm->bo_defer),
> +		 "VM BO cleanup list should be empty.\n");
>  
>  	drm_gem_object_put(gpuvm->r_obj);
>  }
> @@ -1217,6 +1242,9 @@ drm_gpuvm_prepare_objects_locked(struct drm_gpuvm *gpuvm,
>  
>  	drm_gpuvm_resv_assert_held(gpuvm);
>  	list_for_each_entry(vm_bo, &gpuvm->extobj.list, list.entry.extobj) {
> +		if (drm_gpuvm_bo_is_zombie(vm_bo))
> +			continue;
> +
>  		ret = exec_prepare_obj(exec, vm_bo->obj, num_fences);
>  		if (ret)
>  			break;
> @@ -1460,6 +1488,9 @@ drm_gpuvm_validate_locked(struct drm_gpuvm *gpuvm, struct drm_exec *exec)
>  
>  	list_for_each_entry_safe(vm_bo, next, &gpuvm->evict.list,
>  				 list.entry.evict) {
> +		if (drm_gpuvm_bo_is_zombie(vm_bo))
> +			continue;
> +
>  		ret = ops->vm_bo_validate(vm_bo, exec);
>  		if (ret)
>  			break;
> @@ -1560,6 +1591,7 @@ drm_gpuvm_bo_create(struct drm_gpuvm *gpuvm,
>  
>  	INIT_LIST_HEAD(&vm_bo->list.entry.extobj);
>  	INIT_LIST_HEAD(&vm_bo->list.entry.evict);
> +	init_llist_node(&vm_bo->list.entry.bo_defer);
>  
>  	return vm_bo;
>  }
> @@ -1621,6 +1653,124 @@ drm_gpuvm_bo_put(struct drm_gpuvm_bo *vm_bo)
>  }
>  EXPORT_SYMBOL_GPL(drm_gpuvm_bo_put);
>  
> +/*
> + * Must be called with GEM mutex held. After releasing GEM mutex,
> + * drm_gpuvm_bo_defer_free_unlocked() must be called.
> + */
> +static void
> +drm_gpuvm_bo_defer_free_locked(struct kref *kref)
> +{
> +	struct drm_gpuvm_bo *vm_bo = container_of(kref, struct drm_gpuvm_bo,
> +						  kref);
> +	struct drm_gpuvm *gpuvm = vm_bo->vm;
> +
> +	if (!drm_gpuvm_resv_protected(gpuvm)) {
> +		drm_gpuvm_bo_list_del(vm_bo, extobj, true);
> +		drm_gpuvm_bo_list_del(vm_bo, evict, true);
> +	}
> +
> +	list_del(&vm_bo->list.entry.gem);
> +}
> +
> +/*
> + * GEM mutex must not be held. Called after drm_gpuvm_bo_defer_free_locked().
> + */
> +static void
> +drm_gpuvm_bo_defer_free_unlocked(struct drm_gpuvm_bo *vm_bo)
> +{
> +	struct drm_gpuvm *gpuvm = vm_bo->vm;
> +
> +	llist_add(&vm_bo->list.entry.bo_defer, &gpuvm->bo_defer);

Could we simply move this line to drm_gpuvm_bo_defer_free_locked()?
I might be missing something, but I don't really see a reason to
have it exposed as a separate operation.

> +}
> +
> +static void
> +drm_gpuvm_bo_defer_free(struct kref *kref)
> +{
> +	struct drm_gpuvm_bo *vm_bo = container_of(kref, struct drm_gpuvm_bo,
> +						  kref);
> +
> +	mutex_lock(&vm_bo->obj->gpuva.lock);
> +	drm_gpuvm_bo_defer_free_locked(kref);
> +	mutex_unlock(&vm_bo->obj->gpuva.lock);
> +
> +	/*
> +	 * It's important that the GEM stays alive for the duration in which we
> +	 * hold the mutex, but the instant we add the vm_bo to bo_defer,
> +	 * another thread might call drm_gpuvm_bo_deferred_cleanup() and put
> +	 * the GEM. Therefore, to avoid kfreeing a mutex we are holding, we add
> +	 * the vm_bo to bo_defer *after* releasing the GEM's mutex.
> +	 */
> +	drm_gpuvm_bo_defer_free_unlocked(vm_bo);
> +}
> +
> +/**
> + * drm_gpuvm_bo_put_deferred() - drop a struct drm_gpuvm_bo reference with
> + * deferred cleanup
> + * @vm_bo: the &drm_gpuvm_bo to release the reference of
> + *
> + * This releases a reference to @vm_bo.
> + *
> + * This might take and release the GEMs GPUVA lock. You should call
> + * drm_gpuvm_bo_deferred_cleanup() later to complete the cleanup process.
> + *
> + * Returns: true if vm_bo is being destroyed, false otherwise.
> + */
> +bool
> +drm_gpuvm_bo_put_deferred(struct drm_gpuvm_bo *vm_bo)
> +{
> +	if (!vm_bo)
> +		return false;
> +
> +	drm_WARN_ON(vm_bo->vm->drm, !drm_gpuvm_immediate_mode(vm_bo->vm));
> +
> +	return !!kref_put(&vm_bo->kref, drm_gpuvm_bo_defer_free);
> +}
> +EXPORT_SYMBOL_GPL(drm_gpuvm_bo_put_deferred);
> +
> +/**
> + * drm_gpuvm_bo_deferred_cleanup() - clean up BOs in the deferred list
> + * deferred cleanup
> + * @gpuvm: the VM to clean up
> + *
> + * Cleans up &drm_gpuvm_bo instances in the deferred cleanup list.
> + */
> +void
> +drm_gpuvm_bo_deferred_cleanup(struct drm_gpuvm *gpuvm)
> +{
> +	const struct drm_gpuvm_ops *ops = gpuvm->ops;
> +	struct drm_gpuvm_bo *vm_bo;
> +	struct drm_gem_object *obj;
> +	struct llist_node *bo_defer;
> +
> +	bo_defer = llist_del_all(&gpuvm->bo_defer);
> +	if (!bo_defer)
> +		return;
> +
> +	if (drm_gpuvm_resv_protected(gpuvm)) {
> +		dma_resv_lock(drm_gpuvm_resv(gpuvm), NULL);
> +		llist_for_each_entry(vm_bo, bo_defer, list.entry.bo_defer) {
> +			drm_gpuvm_bo_list_del(vm_bo, extobj, false);
> +			drm_gpuvm_bo_list_del(vm_bo, evict, false);
> +		}
> +		dma_resv_unlock(drm_gpuvm_resv(gpuvm));
> +	}
> +
> +	while (bo_defer) {
> +		vm_bo = llist_entry(bo_defer,
> +			struct drm_gpuvm_bo, list.entry.bo_defer);

nit: second line of arguments should be aligned on the open parenthesis.

		vm_bo = llist_entry(bo_defer,
				    struct drm_gpuvm_bo,
				    list.entry.bo_defer);

> +		bo_defer = bo_defer->next;
> +		obj = vm_bo->obj;
> +		if (ops && ops->vm_bo_free)
> +			ops->vm_bo_free(vm_bo);
> +		else
> +			kfree(vm_bo);
> +
> +		drm_gpuvm_put(gpuvm);
> +		drm_gem_object_put(obj);
> +	}
> +}
> +EXPORT_SYMBOL_GPL(drm_gpuvm_bo_deferred_cleanup);
> +
>  static struct drm_gpuvm_bo *
>  __drm_gpuvm_bo_find(struct drm_gpuvm *gpuvm,
>  		    struct drm_gem_object *obj)
> @@ -1948,6 +2098,40 @@ drm_gpuva_unlink(struct drm_gpuva *va)
>  }
>  EXPORT_SYMBOL_GPL(drm_gpuva_unlink);
>  
> +/**
> + * drm_gpuva_unlink_defer() - unlink a &drm_gpuva with deferred vm_bo cleanup
> + * @va: the &drm_gpuva to unlink
> + *
> + * Similar to drm_gpuva_unlink(), but uses drm_gpuvm_bo_put_deferred() and takes
> + * the lock for the caller.
> + */
> +void
> +drm_gpuva_unlink_defer(struct drm_gpuva *va)
> +{
> +	struct drm_gem_object *obj = va->gem.obj;
> +	struct drm_gpuvm_bo *vm_bo = va->vm_bo;
> +	bool should_defer_bo;
> +
> +	if (unlikely(!obj))
> +		return;
> +
> +	drm_WARN_ON(vm_bo->vm->drm, !drm_gpuvm_immediate_mode(vm_bo->vm));
> +
> +	mutex_lock(&obj->gpuva.lock);
> +	list_del_init(&va->gem.entry);
> +
> +	/*
> +	 * This is drm_gpuvm_bo_put_deferred() except we already hold the mutex.
> +	 */
> +	should_defer_bo = kref_put(&vm_bo->kref, drm_gpuvm_bo_defer_free_locked);
> +	mutex_unlock(&obj->gpuva.lock);
> +	if (should_defer_bo)
> +		drm_gpuvm_bo_defer_free_unlocked(vm_bo);
> +
> +	va->vm_bo = NULL;
> +}
> +EXPORT_SYMBOL_GPL(drm_gpuva_unlink_defer);
> +
>  /**
>   * drm_gpuva_find_first() - find the first &drm_gpuva in the given range
>   * @gpuvm: the &drm_gpuvm to search in
> diff --git a/include/drm/drm_gpuvm.h b/include/drm/drm_gpuvm.h
> index 8890ded1d90752a2acbb564f697aa5ab03b5d052..81cc7672cf2d5362c637abfa2a75471e5274ed08 100644
> --- a/include/drm/drm_gpuvm.h
> +++ b/include/drm/drm_gpuvm.h
> @@ -27,6 +27,7 @@
>  
>  #include <linux/dma-resv.h>
>  #include <linux/list.h>
> +#include <linux/llist.h>
>  #include <linux/rbtree.h>
>  #include <linux/types.h>
>  
> @@ -152,6 +153,7 @@ void drm_gpuva_remove(struct drm_gpuva *va);
>  
>  void drm_gpuva_link(struct drm_gpuva *va, struct drm_gpuvm_bo *vm_bo);
>  void drm_gpuva_unlink(struct drm_gpuva *va);
> +void drm_gpuva_unlink_defer(struct drm_gpuva *va);
>  
>  struct drm_gpuva *drm_gpuva_find(struct drm_gpuvm *gpuvm,
>  				 u64 addr, u64 range);
> @@ -331,6 +333,11 @@ struct drm_gpuvm {
>  		 */
>  		spinlock_t lock;
>  	} evict;
> +
> +	/**
> +	 * @bo_defer: structure holding vm_bos that need to be destroyed
> +	 */
> +	struct llist_head bo_defer;
>  };
>  
>  void drm_gpuvm_init(struct drm_gpuvm *gpuvm, const char *name,
> @@ -714,6 +721,12 @@ struct drm_gpuvm_bo {
>  			 * &drm_gpuvms evict list.
>  			 */
>  			struct list_head evict;
> +
> +			/**
> +			 * @list.entry.bo_defer: List entry to attach to
> +			 * the &drm_gpuvms bo_defer list.
> +			 */
> +			struct llist_node bo_defer;
>  		} entry;
>  	} list;
>  };
> @@ -746,6 +759,9 @@ drm_gpuvm_bo_get(struct drm_gpuvm_bo *vm_bo)
>  
>  bool drm_gpuvm_bo_put(struct drm_gpuvm_bo *vm_bo);
>  
> +bool drm_gpuvm_bo_put_deferred(struct drm_gpuvm_bo *vm_bo);
> +void drm_gpuvm_bo_deferred_cleanup(struct drm_gpuvm *gpuvm);
> +
>  struct drm_gpuvm_bo *
>  drm_gpuvm_bo_find(struct drm_gpuvm *gpuvm,
>  		  struct drm_gem_object *obj);
> 

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