[07/26] Revert "drm/i915/gem: Split eb_vma into its own allocation"
diff mbox series

Message ID 20200623142843.423594-7-maarten.lankhorst@linux.intel.com
State New
Headers show
Series
  • [01/26] Revert "drm/i915/gem: Async GPU relocations only"
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Commit Message

Maarten Lankhorst June 23, 2020, 2:28 p.m. UTC
This reverts commit 0f1dd02295f35dcdcbaafcbcbbec0753884ab974.
This conflicts with the ww mutex handling, which needs to drop
the references after gpu submission anyway, because otherwise we
may risk unlocking a BO after first freeing it.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
---
 .../gpu/drm/i915/gem/i915_gem_execbuffer.c    | 124 +++++++-----------
 1 file changed, 51 insertions(+), 73 deletions(-)

Comments

Tvrtko Ursulin June 29, 2020, 3:08 p.m. UTC | #1
On 23/06/2020 15:28, Maarten Lankhorst wrote:
> This reverts commit 0f1dd02295f35dcdcbaafcbcbbec0753884ab974.
> This conflicts with the ww mutex handling, which needs to drop
> the references after gpu submission anyway, because otherwise we
> may risk unlocking a BO after first freeing it.

What is the problem here? eb_vma_array_put in eb_move_to_gpu? If so, 
could you just move this put to later in the sequence? I am simply 
thinking how to avoid controversial reverts. Because on the other hand I 
did not figure out what 0f1dd02295f35dcdcbaafcbcbbec0753884ab974 fixed 
in a few minutes I spent staring at the patch.

Regards,

Tvrtko

> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> ---
>   .../gpu/drm/i915/gem/i915_gem_execbuffer.c    | 124 +++++++-----------
>   1 file changed, 51 insertions(+), 73 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> index 7cb44915cfc7..2636a130fb57 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> @@ -40,11 +40,6 @@ struct eb_vma {
>   	u32 handle;
>   };
>   
> -struct eb_vma_array {
> -	struct kref kref;
> -	struct eb_vma vma[];
> -};
> -
>   enum {
>   	FORCE_CPU_RELOC = 1,
>   	FORCE_GTT_RELOC,
> @@ -57,6 +52,7 @@ enum {
>   #define __EXEC_OBJECT_NEEDS_MAP		BIT(29)
>   #define __EXEC_OBJECT_NEEDS_BIAS	BIT(28)
>   #define __EXEC_OBJECT_INTERNAL_FLAGS	(~0u << 28) /* all of the above */
> +#define __EXEC_OBJECT_RESERVED (__EXEC_OBJECT_HAS_PIN | __EXEC_OBJECT_HAS_FENCE)
>   
>   #define __EXEC_HAS_RELOC	BIT(31)
>   #define __EXEC_INTERNAL_FLAGS	(~0u << 31)
> @@ -287,7 +283,6 @@ struct i915_execbuffer {
>   	 */
>   	int lut_size;
>   	struct hlist_head *buckets; /** ht for relocation handles */
> -	struct eb_vma_array *array;
>   };
>   
>   static int eb_parse(struct i915_execbuffer *eb);
> @@ -299,62 +294,8 @@ static inline bool eb_use_cmdparser(const struct i915_execbuffer *eb)
>   		 eb->args->batch_len);
>   }
>   
> -static struct eb_vma_array *eb_vma_array_create(unsigned int count)
> -{
> -	struct eb_vma_array *arr;
> -
> -	arr = kvmalloc(struct_size(arr, vma, count), GFP_KERNEL | __GFP_NOWARN);
> -	if (!arr)
> -		return NULL;
> -
> -	kref_init(&arr->kref);
> -	arr->vma[0].vma = NULL;
> -
> -	return arr;
> -}
> -
> -static inline void eb_unreserve_vma(struct eb_vma *ev)
> -{
> -	struct i915_vma *vma = ev->vma;
> -
> -	if (unlikely(ev->flags & __EXEC_OBJECT_HAS_FENCE))
> -		__i915_vma_unpin_fence(vma);
> -
> -	if (ev->flags & __EXEC_OBJECT_HAS_PIN)
> -		__i915_vma_unpin(vma);
> -
> -	ev->flags &= ~(__EXEC_OBJECT_HAS_PIN |
> -		       __EXEC_OBJECT_HAS_FENCE);
> -}
> -
> -static void eb_vma_array_destroy(struct kref *kref)
> -{
> -	struct eb_vma_array *arr = container_of(kref, typeof(*arr), kref);
> -	struct eb_vma *ev = arr->vma;
> -
> -	while (ev->vma) {
> -		eb_unreserve_vma(ev);
> -		i915_vma_put(ev->vma);
> -		ev++;
> -	}
> -
> -	kvfree(arr);
> -}
> -
> -static void eb_vma_array_put(struct eb_vma_array *arr)
> -{
> -	kref_put(&arr->kref, eb_vma_array_destroy);
> -}
> -
>   static int eb_create(struct i915_execbuffer *eb)
>   {
> -	/* Allocate an extra slot for use by the command parser + sentinel */
> -	eb->array = eb_vma_array_create(eb->buffer_count + 2);
> -	if (!eb->array)
> -		return -ENOMEM;
> -
> -	eb->vma = eb->array->vma;
> -
>   	if (!(eb->args->flags & I915_EXEC_HANDLE_LUT)) {
>   		unsigned int size = 1 + ilog2(eb->buffer_count);
>   
> @@ -388,10 +329,8 @@ static int eb_create(struct i915_execbuffer *eb)
>   				break;
>   		} while (--size);
>   
> -		if (unlikely(!size)) {
> -			eb_vma_array_put(eb->array);
> +		if (unlikely(!size))
>   			return -ENOMEM;
> -		}
>   
>   		eb->lut_size = size;
>   	} else {
> @@ -502,6 +441,26 @@ eb_pin_vma(struct i915_execbuffer *eb,
>   	return !eb_vma_misplaced(entry, vma, ev->flags);
>   }
>   
> +static inline void __eb_unreserve_vma(struct i915_vma *vma, unsigned int flags)
> +{
> +	GEM_BUG_ON(!(flags & __EXEC_OBJECT_HAS_PIN));
> +
> +	if (unlikely(flags & __EXEC_OBJECT_HAS_FENCE))
> +		__i915_vma_unpin_fence(vma);
> +
> +	__i915_vma_unpin(vma);
> +}
> +
> +static inline void
> +eb_unreserve_vma(struct eb_vma *ev)
> +{
> +	if (!(ev->flags & __EXEC_OBJECT_HAS_PIN))
> +		return;
> +
> +	__eb_unreserve_vma(ev->vma, ev->flags);
> +	ev->flags &= ~__EXEC_OBJECT_RESERVED;
> +}
> +
>   static int
>   eb_validate_vma(struct i915_execbuffer *eb,
>   		struct drm_i915_gem_exec_object2 *entry,
> @@ -944,13 +903,31 @@ eb_get_vma(const struct i915_execbuffer *eb, unsigned long handle)
>   	}
>   }
>   
> +static void eb_release_vmas(const struct i915_execbuffer *eb)
> +{
> +	const unsigned int count = eb->buffer_count;
> +	unsigned int i;
> +
> +	for (i = 0; i < count; i++) {
> +		struct eb_vma *ev = &eb->vma[i];
> +		struct i915_vma *vma = ev->vma;
> +
> +		if (!vma)
> +			break;
> +
> +		eb->vma[i].vma = NULL;
> +
> +		if (ev->flags & __EXEC_OBJECT_HAS_PIN)
> +			__eb_unreserve_vma(vma, ev->flags);
> +
> +		i915_vma_put(vma);
> +	}
> +}
> +
>   static void eb_destroy(const struct i915_execbuffer *eb)
>   {
>   	GEM_BUG_ON(eb->reloc_cache.rq);
>   
> -	if (eb->array)
> -		eb_vma_array_put(eb->array);
> -
>   	if (eb->lut_size > 0)
>   		kfree(eb->buckets);
>   }
> @@ -2039,12 +2016,9 @@ static int eb_move_to_gpu(struct i915_execbuffer *eb)
>   			err = i915_vma_move_to_active(vma, eb->request, flags);
>   
>   		i915_vma_unlock(vma);
> -		eb_unreserve_vma(ev);
>   	}
>   	ww_acquire_fini(&acquire);
>   
> -	eb_vma_array_put(fetch_and_zero(&eb->array));
> -
>   	if (unlikely(err))
>   		goto err_skip;
>   
> @@ -2340,7 +2314,6 @@ static int eb_parse(struct i915_execbuffer *eb)
>   	eb->vma[eb->buffer_count].vma = i915_vma_get(shadow);
>   	eb->vma[eb->buffer_count].flags = __EXEC_OBJECT_HAS_PIN;
>   	eb->batch = &eb->vma[eb->buffer_count++];
> -	eb->vma[eb->buffer_count].vma = NULL;
>   
>   	eb->trampoline = trampoline;
>   	eb->batch_start_offset = 0;
> @@ -2838,6 +2811,8 @@ i915_gem_do_execbuffer(struct drm_device *dev,
>   		args->flags |= __EXEC_HAS_RELOC;
>   
>   	eb.exec = exec;
> +	eb.vma = (struct eb_vma *)(exec + args->buffer_count + 1);
> +	eb.vma[0].vma = NULL;
>   
>   	eb.invalid_flags = __EXEC_OBJECT_UNKNOWN_FLAGS;
>   	reloc_cache_init(&eb.reloc_cache, eb.i915);
> @@ -3014,6 +2989,8 @@ i915_gem_do_execbuffer(struct drm_device *dev,
>   	if (batch->private)
>   		intel_gt_buffer_pool_put(batch->private);
>   err_vma:
> +	if (eb.exec)
> +		eb_release_vmas(&eb);
>   	if (eb.trampoline)
>   		i915_vma_unpin(eb.trampoline);
>   	eb_unpin_engine(&eb);
> @@ -3031,7 +3008,7 @@ i915_gem_do_execbuffer(struct drm_device *dev,
>   
>   static size_t eb_element_size(void)
>   {
> -	return sizeof(struct drm_i915_gem_exec_object2);
> +	return sizeof(struct drm_i915_gem_exec_object2) + sizeof(struct eb_vma);
>   }
>   
>   static bool check_buffer_count(size_t count)
> @@ -3087,7 +3064,7 @@ i915_gem_execbuffer_ioctl(struct drm_device *dev, void *data,
>   	/* Copy in the exec list from userland */
>   	exec_list = kvmalloc_array(count, sizeof(*exec_list),
>   				   __GFP_NOWARN | GFP_KERNEL);
> -	exec2_list = kvmalloc_array(count, eb_element_size(),
> +	exec2_list = kvmalloc_array(count + 1, eb_element_size(),
>   				    __GFP_NOWARN | GFP_KERNEL);
>   	if (exec_list == NULL || exec2_list == NULL) {
>   		drm_dbg(&i915->drm,
> @@ -3165,7 +3142,8 @@ i915_gem_execbuffer2_ioctl(struct drm_device *dev, void *data,
>   	if (err)
>   		return err;
>   
> -	exec2_list = kvmalloc_array(count, eb_element_size(),
> +	/* Allocate an extra slot for use by the command parser */
> +	exec2_list = kvmalloc_array(count + 1, eb_element_size(),
>   				    __GFP_NOWARN | GFP_KERNEL);
>   	if (exec2_list == NULL) {
>   		drm_dbg(&i915->drm, "Failed to allocate exec list for %zd buffers\n",
>
Maarten Lankhorst June 30, 2020, 11:52 a.m. UTC | #2
Op 29-06-2020 om 17:08 schreef Tvrtko Ursulin:
>
> On 23/06/2020 15:28, Maarten Lankhorst wrote:
>> This reverts commit 0f1dd02295f35dcdcbaafcbcbbec0753884ab974.
>> This conflicts with the ww mutex handling, which needs to drop
>> the references after gpu submission anyway, because otherwise we
>> may risk unlocking a BO after first freeing it.
>
> What is the problem here? eb_vma_array_put in eb_move_to_gpu? If so, could you just move this put to later in the sequence? I am simply thinking how to avoid controversial reverts. Because on the other hand I did not figure out what 0f1dd02295f35dcdcbaafcbcbbec0753884ab974 fixed in a few minutes I spent staring at the patch. 


We need to unlock before we unref to prevent a use-after-free in unlock, so freeing and releasing in eb_move_to_gpu() is too early. This means we only end up with 1 path for unlock, so it's fine to revert.
Tvrtko Ursulin June 30, 2020, 12:31 p.m. UTC | #3
On 30/06/2020 12:52, Maarten Lankhorst wrote:
> Op 29-06-2020 om 17:08 schreef Tvrtko Ursulin:
>>
>> On 23/06/2020 15:28, Maarten Lankhorst wrote:
>>> This reverts commit 0f1dd02295f35dcdcbaafcbcbbec0753884ab974.
>>> This conflicts with the ww mutex handling, which needs to drop
>>> the references after gpu submission anyway, because otherwise we
>>> may risk unlocking a BO after first freeing it.
>>
>> What is the problem here? eb_vma_array_put in eb_move_to_gpu? If so, could you just move this put to later in the sequence? I am simply thinking how to avoid controversial reverts. Because on the other hand I did not figure out what 0f1dd02295f35dcdcbaafcbcbbec0753884ab974 fixed in a few minutes I spent staring at the patch.
> 
> 
> We need to unlock before we unref to prevent a use-after-free in unlock, so freeing and releasing in eb_move_to_gpu() is too early. This means we only end up with 1 path for unlock, so it's fine to revert.

You are saying the reason 0f1dd02295f35dcdcbaafcbcbbec0753884ab974 was 
added for will not be there after your changes?

Regards,

Tvrtko
Maarten Lankhorst June 30, 2020, 2:07 p.m. UTC | #4
Op 30-06-2020 om 14:31 schreef Tvrtko Ursulin:
>
> On 30/06/2020 12:52, Maarten Lankhorst wrote:
>> Op 29-06-2020 om 17:08 schreef Tvrtko Ursulin:
>>>
>>> On 23/06/2020 15:28, Maarten Lankhorst wrote:
>>>> This reverts commit 0f1dd02295f35dcdcbaafcbcbbec0753884ab974.
>>>> This conflicts with the ww mutex handling, which needs to drop
>>>> the references after gpu submission anyway, because otherwise we
>>>> may risk unlocking a BO after first freeing it.
>>>
>>> What is the problem here? eb_vma_array_put in eb_move_to_gpu? If so, could you just move this put to later in the sequence? I am simply thinking how to avoid controversial reverts. Because on the other hand I did not figure out what 0f1dd02295f35dcdcbaafcbcbbec0753884ab974 fixed in a few minutes I spent staring at the patch.
>>
>>
>> We need to unlock before we unref to prevent a use-after-free in unlock, so freeing and releasing in eb_move_to_gpu() is too early. This means we only end up with 1 path for unlock, so it's fine to revert.
>
> You are saying the reason 0f1dd02295f35dcdcbaafcbcbbec0753884ab974 was added for will not be there after your changes?
>
> Regards,
>
> Tvrtko

Yes. :)

Patch
diff mbox series

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index 7cb44915cfc7..2636a130fb57 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -40,11 +40,6 @@  struct eb_vma {
 	u32 handle;
 };
 
-struct eb_vma_array {
-	struct kref kref;
-	struct eb_vma vma[];
-};
-
 enum {
 	FORCE_CPU_RELOC = 1,
 	FORCE_GTT_RELOC,
@@ -57,6 +52,7 @@  enum {
 #define __EXEC_OBJECT_NEEDS_MAP		BIT(29)
 #define __EXEC_OBJECT_NEEDS_BIAS	BIT(28)
 #define __EXEC_OBJECT_INTERNAL_FLAGS	(~0u << 28) /* all of the above */
+#define __EXEC_OBJECT_RESERVED (__EXEC_OBJECT_HAS_PIN | __EXEC_OBJECT_HAS_FENCE)
 
 #define __EXEC_HAS_RELOC	BIT(31)
 #define __EXEC_INTERNAL_FLAGS	(~0u << 31)
@@ -287,7 +283,6 @@  struct i915_execbuffer {
 	 */
 	int lut_size;
 	struct hlist_head *buckets; /** ht for relocation handles */
-	struct eb_vma_array *array;
 };
 
 static int eb_parse(struct i915_execbuffer *eb);
@@ -299,62 +294,8 @@  static inline bool eb_use_cmdparser(const struct i915_execbuffer *eb)
 		 eb->args->batch_len);
 }
 
-static struct eb_vma_array *eb_vma_array_create(unsigned int count)
-{
-	struct eb_vma_array *arr;
-
-	arr = kvmalloc(struct_size(arr, vma, count), GFP_KERNEL | __GFP_NOWARN);
-	if (!arr)
-		return NULL;
-
-	kref_init(&arr->kref);
-	arr->vma[0].vma = NULL;
-
-	return arr;
-}
-
-static inline void eb_unreserve_vma(struct eb_vma *ev)
-{
-	struct i915_vma *vma = ev->vma;
-
-	if (unlikely(ev->flags & __EXEC_OBJECT_HAS_FENCE))
-		__i915_vma_unpin_fence(vma);
-
-	if (ev->flags & __EXEC_OBJECT_HAS_PIN)
-		__i915_vma_unpin(vma);
-
-	ev->flags &= ~(__EXEC_OBJECT_HAS_PIN |
-		       __EXEC_OBJECT_HAS_FENCE);
-}
-
-static void eb_vma_array_destroy(struct kref *kref)
-{
-	struct eb_vma_array *arr = container_of(kref, typeof(*arr), kref);
-	struct eb_vma *ev = arr->vma;
-
-	while (ev->vma) {
-		eb_unreserve_vma(ev);
-		i915_vma_put(ev->vma);
-		ev++;
-	}
-
-	kvfree(arr);
-}
-
-static void eb_vma_array_put(struct eb_vma_array *arr)
-{
-	kref_put(&arr->kref, eb_vma_array_destroy);
-}
-
 static int eb_create(struct i915_execbuffer *eb)
 {
-	/* Allocate an extra slot for use by the command parser + sentinel */
-	eb->array = eb_vma_array_create(eb->buffer_count + 2);
-	if (!eb->array)
-		return -ENOMEM;
-
-	eb->vma = eb->array->vma;
-
 	if (!(eb->args->flags & I915_EXEC_HANDLE_LUT)) {
 		unsigned int size = 1 + ilog2(eb->buffer_count);
 
@@ -388,10 +329,8 @@  static int eb_create(struct i915_execbuffer *eb)
 				break;
 		} while (--size);
 
-		if (unlikely(!size)) {
-			eb_vma_array_put(eb->array);
+		if (unlikely(!size))
 			return -ENOMEM;
-		}
 
 		eb->lut_size = size;
 	} else {
@@ -502,6 +441,26 @@  eb_pin_vma(struct i915_execbuffer *eb,
 	return !eb_vma_misplaced(entry, vma, ev->flags);
 }
 
+static inline void __eb_unreserve_vma(struct i915_vma *vma, unsigned int flags)
+{
+	GEM_BUG_ON(!(flags & __EXEC_OBJECT_HAS_PIN));
+
+	if (unlikely(flags & __EXEC_OBJECT_HAS_FENCE))
+		__i915_vma_unpin_fence(vma);
+
+	__i915_vma_unpin(vma);
+}
+
+static inline void
+eb_unreserve_vma(struct eb_vma *ev)
+{
+	if (!(ev->flags & __EXEC_OBJECT_HAS_PIN))
+		return;
+
+	__eb_unreserve_vma(ev->vma, ev->flags);
+	ev->flags &= ~__EXEC_OBJECT_RESERVED;
+}
+
 static int
 eb_validate_vma(struct i915_execbuffer *eb,
 		struct drm_i915_gem_exec_object2 *entry,
@@ -944,13 +903,31 @@  eb_get_vma(const struct i915_execbuffer *eb, unsigned long handle)
 	}
 }
 
+static void eb_release_vmas(const struct i915_execbuffer *eb)
+{
+	const unsigned int count = eb->buffer_count;
+	unsigned int i;
+
+	for (i = 0; i < count; i++) {
+		struct eb_vma *ev = &eb->vma[i];
+		struct i915_vma *vma = ev->vma;
+
+		if (!vma)
+			break;
+
+		eb->vma[i].vma = NULL;
+
+		if (ev->flags & __EXEC_OBJECT_HAS_PIN)
+			__eb_unreserve_vma(vma, ev->flags);
+
+		i915_vma_put(vma);
+	}
+}
+
 static void eb_destroy(const struct i915_execbuffer *eb)
 {
 	GEM_BUG_ON(eb->reloc_cache.rq);
 
-	if (eb->array)
-		eb_vma_array_put(eb->array);
-
 	if (eb->lut_size > 0)
 		kfree(eb->buckets);
 }
@@ -2039,12 +2016,9 @@  static int eb_move_to_gpu(struct i915_execbuffer *eb)
 			err = i915_vma_move_to_active(vma, eb->request, flags);
 
 		i915_vma_unlock(vma);
-		eb_unreserve_vma(ev);
 	}
 	ww_acquire_fini(&acquire);
 
-	eb_vma_array_put(fetch_and_zero(&eb->array));
-
 	if (unlikely(err))
 		goto err_skip;
 
@@ -2340,7 +2314,6 @@  static int eb_parse(struct i915_execbuffer *eb)
 	eb->vma[eb->buffer_count].vma = i915_vma_get(shadow);
 	eb->vma[eb->buffer_count].flags = __EXEC_OBJECT_HAS_PIN;
 	eb->batch = &eb->vma[eb->buffer_count++];
-	eb->vma[eb->buffer_count].vma = NULL;
 
 	eb->trampoline = trampoline;
 	eb->batch_start_offset = 0;
@@ -2838,6 +2811,8 @@  i915_gem_do_execbuffer(struct drm_device *dev,
 		args->flags |= __EXEC_HAS_RELOC;
 
 	eb.exec = exec;
+	eb.vma = (struct eb_vma *)(exec + args->buffer_count + 1);
+	eb.vma[0].vma = NULL;
 
 	eb.invalid_flags = __EXEC_OBJECT_UNKNOWN_FLAGS;
 	reloc_cache_init(&eb.reloc_cache, eb.i915);
@@ -3014,6 +2989,8 @@  i915_gem_do_execbuffer(struct drm_device *dev,
 	if (batch->private)
 		intel_gt_buffer_pool_put(batch->private);
 err_vma:
+	if (eb.exec)
+		eb_release_vmas(&eb);
 	if (eb.trampoline)
 		i915_vma_unpin(eb.trampoline);
 	eb_unpin_engine(&eb);
@@ -3031,7 +3008,7 @@  i915_gem_do_execbuffer(struct drm_device *dev,
 
 static size_t eb_element_size(void)
 {
-	return sizeof(struct drm_i915_gem_exec_object2);
+	return sizeof(struct drm_i915_gem_exec_object2) + sizeof(struct eb_vma);
 }
 
 static bool check_buffer_count(size_t count)
@@ -3087,7 +3064,7 @@  i915_gem_execbuffer_ioctl(struct drm_device *dev, void *data,
 	/* Copy in the exec list from userland */
 	exec_list = kvmalloc_array(count, sizeof(*exec_list),
 				   __GFP_NOWARN | GFP_KERNEL);
-	exec2_list = kvmalloc_array(count, eb_element_size(),
+	exec2_list = kvmalloc_array(count + 1, eb_element_size(),
 				    __GFP_NOWARN | GFP_KERNEL);
 	if (exec_list == NULL || exec2_list == NULL) {
 		drm_dbg(&i915->drm,
@@ -3165,7 +3142,8 @@  i915_gem_execbuffer2_ioctl(struct drm_device *dev, void *data,
 	if (err)
 		return err;
 
-	exec2_list = kvmalloc_array(count, eb_element_size(),
+	/* Allocate an extra slot for use by the command parser */
+	exec2_list = kvmalloc_array(count + 1, eb_element_size(),
 				    __GFP_NOWARN | GFP_KERNEL);
 	if (exec2_list == NULL) {
 		drm_dbg(&i915->drm, "Failed to allocate exec list for %zd buffers\n",