[PATCH] drm/i915: Always mark the object as dirty when used by the GPU

[PATCH] drm/i915: Always mark the object as dirty when used by the GPU

[Chris Wilson]

There have been many hard to track down bugs whereby userspace forgot to
flag a write buffer and then cause graphics corruption or a hung GPU
when that buffer was later purged under memory pressure (as the buffer
appeared clean, its pages would have been evicted rather than preserved
and any changes more recent than in the backing storage would be lost).
In retrospect this is a rare optimisation against memory pressure,
already the slow path. If we always mark the buffer as dirty when
accessed by the GPU, anything not used can still be evicted cheaply
(ideal behaviour for mark-and-sweep eviction) but we do not run the risk
of corruption. For correct read serialisation, userspace still has to
notify when the GPU writes to an object. However, there are certain
situations under which userspace may wish to tell white lies to the
kernel...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Kristian Høgsberg <krh@bitplanet.net>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: "Goel, Akash" <akash.goel@intel.co>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: stable@vger.kernel.org
---
 drivers/gpu/drm/i915/i915_gem_execbuffer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 923a3c4bf0b7..a953d4975b8c 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1032,6 +1032,7 @@ i915_gem_execbuffer_move_to_active(struct list_head *vmas,
 		u32 old_read = obj->base.read_domains;
 		u32 old_write = obj->base.write_domain;
 
+		obj->dirty = 1; /* be paranoid  */
 		obj->base.write_domain = obj->base.pending_write_domain;
 		if (obj->base.write_domain == 0)
 			obj->base.pending_read_domains |= obj->base.read_domains;
@@ -1039,7 +1040,6 @@ i915_gem_execbuffer_move_to_active(struct list_head *vmas,
 
 		i915_vma_move_to_active(vma, req);
 		if (obj->base.write_domain) {
-			obj->dirty = 1;
 			i915_gem_request_assign(&obj->last_write_req, req);
 
 			intel_fb_obj_invalidate(obj, ORIGIN_CS);
-- 
2.5.1

Re: [PATCH] drm/i915: Always mark the object as dirty when used by the GPU

[Daniel Vetter]

On Mon, Aug 31, 2015 at 03:10:39PM +0100, Chris Wilson wrote:
> There have been many hard to track down bugs whereby userspace forgot to
> flag a write buffer and then cause graphics corruption or a hung GPU
> when that buffer was later purged under memory pressure (as the buffer
> appeared clean, its pages would have been evicted rather than preserved
> and any changes more recent than in the backing storage would be lost).
> In retrospect this is a rare optimisation against memory pressure,
> already the slow path. If we always mark the buffer as dirty when
> accessed by the GPU, anything not used can still be evicted cheaply
> (ideal behaviour for mark-and-sweep eviction) but we do not run the risk
> of corruption. For correct read serialisation, userspace still has to
> notify when the GPU writes to an object. However, there are certain
> situations under which userspace may wish to tell white lies to the
> kernel...
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Kristian Høgsberg <krh@bitplanet.net>
> Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
> Cc: "Goel, Akash" <akash.goel@intel.co>
> Cc: Michał Winiarski <michal.winiarski@intel.com>
> Cc: stable@vger.kernel.org

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> ---
>  drivers/gpu/drm/i915/i915_gem_execbuffer.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index 923a3c4bf0b7..a953d4975b8c 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -1032,6 +1032,7 @@ i915_gem_execbuffer_move_to_active(struct list_head *vmas,
>  		u32 old_read = obj->base.read_domains;
>  		u32 old_write = obj->base.write_domain;
>  
> +		obj->dirty = 1; /* be paranoid  */
>  		obj->base.write_domain = obj->base.pending_write_domain;
>  		if (obj->base.write_domain == 0)
>  			obj->base.pending_read_domains |= obj->base.read_domains;
> @@ -1039,7 +1040,6 @@ i915_gem_execbuffer_move_to_active(struct list_head *vmas,
>  
>  		i915_vma_move_to_active(vma, req);
>  		if (obj->base.write_domain) {
> -			obj->dirty = 1;
>  			i915_gem_request_assign(&obj->last_write_req, req);
>  
>  			intel_fb_obj_invalidate(obj, ORIGIN_CS);
> -- 
> 2.5.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH] drm/i915: Always mark the object as dirty when used by the GPU

[Jani Nikula]

On Wed, 02 Sep 2015, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Mon, Aug 31, 2015 at 03:10:39PM +0100, Chris Wilson wrote:
>> There have been many hard to track down bugs whereby userspace forgot to
>> flag a write buffer and then cause graphics corruption or a hung GPU
>> when that buffer was later purged under memory pressure (as the buffer
>> appeared clean, its pages would have been evicted rather than preserved
>> and any changes more recent than in the backing storage would be lost).
>> In retrospect this is a rare optimisation against memory pressure,
>> already the slow path. If we always mark the buffer as dirty when
>> accessed by the GPU, anything not used can still be evicted cheaply
>> (ideal behaviour for mark-and-sweep eviction) but we do not run the risk
>> of corruption. For correct read serialisation, userspace still has to
>> notify when the GPU writes to an object. However, there are certain
>> situations under which userspace may wish to tell white lies to the
>> kernel...
>> 
>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>> Cc: Kristian Høgsberg <krh@bitplanet.net>
>> Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
>> Cc: "Goel, Akash" <akash.goel@intel.co>
>> Cc: Michał Winiarski <michal.winiarski@intel.com>
>> Cc: stable@vger.kernel.org
>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Pushed to drm-intel-next-fixes, thanks for the patch and review.

BR,
Jani.

>
>> ---
>>  drivers/gpu/drm/i915/i915_gem_execbuffer.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
>> index 923a3c4bf0b7..a953d4975b8c 100644
>> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
>> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
>> @@ -1032,6 +1032,7 @@ i915_gem_execbuffer_move_to_active(struct list_head *vmas,
>>  		u32 old_read = obj->base.read_domains;
>>  		u32 old_write = obj->base.write_domain;
>>  
>> +		obj->dirty = 1; /* be paranoid  */
>>  		obj->base.write_domain = obj->base.pending_write_domain;
>>  		if (obj->base.write_domain == 0)
>>  			obj->base.pending_read_domains |= obj->base.read_domains;
>> @@ -1039,7 +1040,6 @@ i915_gem_execbuffer_move_to_active(struct list_head *vmas,
>>  
>>  		i915_vma_move_to_active(vma, req);
>>  		if (obj->base.write_domain) {
>> -			obj->dirty = 1;
>>  			i915_gem_request_assign(&obj->last_write_req, req);
>>  
>>  			intel_fb_obj_invalidate(obj, ORIGIN_CS);
>> -- 
>> 2.5.1
>> 
>
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Jani Nikula, Intel Open Source Technology Center