Re: [PATCH v4 04/13] drm/shmem-helper: Provide a vmap function for short-term mappings

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 11.01.21 um 17:50 schrieb Daniel Vetter:
> On Fri, Jan 08, 2021 at 10:43:31AM +0100, Thomas Zimmermann wrote:
>> Implementations of the vmap/vunmap GEM callbacks may perform pinning
>> of the BO and may acquire the associated reservation object's lock.
>> Callers that only require a mapping of the contained memory can thus
>> interfere with other tasks that require exact pinning, such as scanout.
>> This is less of an issue with private SHMEM buffers, but may happen
>> with imported ones.
>>
>> Therefore provide the new interfaces drm_gem_shmem_vmap_local() and
>> drm_gem_shmem_vunmap_local(), which only perform the vmap/vunmap
>> operations. Callers have to hold the reservation lock while the mapping
>> persists.
>>
>> This patch also connects GEM SHMEM helpers to GEM object functions with
>> equivalent functionality.
>>
>> v4:
>> 	* call dma_buf_{vmap,vunmap}_local() where necessary (Daniel)
>> 	* move driver changes into separate patches (Daniel)
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>> ---
>>   drivers/gpu/drm/drm_gem_shmem_helper.c | 90 +++++++++++++++++++++++---
>>   include/drm/drm_gem_shmem_helper.h     |  2 +
>>   2 files changed, 84 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> index 9825c378dfa6..298832b2b43b 100644
>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> @@ -32,6 +32,8 @@ static const struct drm_gem_object_funcs drm_gem_shmem_funcs = {
>>   	.get_sg_table = drm_gem_shmem_get_sg_table,
>>   	.vmap = drm_gem_shmem_vmap,
>>   	.vunmap = drm_gem_shmem_vunmap,
>> +	.vmap_local = drm_gem_shmem_vmap_local,
>> +	.vunmap_local = drm_gem_shmem_vunmap_local,
>>   	.mmap = drm_gem_shmem_mmap,
>>   };
>>   
>> @@ -261,7 +263,8 @@ void drm_gem_shmem_unpin(struct drm_gem_object *obj)
>>   }
>>   EXPORT_SYMBOL(drm_gem_shmem_unpin);
>>   
>> -static int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem, struct dma_buf_map *map)
>> +static int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem, struct dma_buf_map *map,
>> +				     bool local)
> 
> This is a bit spaghetti and also has the problem that we're not changing
> shmem->vmap_use_count under different locks, depending upon which path
> we're taking.
> 
> I think the cleanest would be if we pull the if (import_attach) case out
> of the _locked() version completely, for all cases, and also outside of
> the shmem->vmap_lock. This means no caching of vmaps in the shmem layer
> anymore for imported buffers, but this is no longer a problem: We cache
> them in the exporters instead (I think at least, if not maybe need to fix
> that where it's expensive).

If we do that, what protects shmem->vaddr from concurrent access near 
line 281? would it be kept NULL then?

Also, we have some stats in debugfs (see drm_gem_shmem_print_info) which 
would be incorrect (or misleading at least).

Given all that, would it be possible to remove vmap_lock in favor of 
taking the resv lock in vmap/vunmap?

Best regards
Thomas

> 
> Other option would be to unly pull it out for the _vmap_local case, but
> that's a bit ugly because no longer symmetrical in the various paths.
> 
>>   {
>>   	struct drm_gem_object *obj = &shmem->base;
>>   	int ret = 0;
>> @@ -272,7 +275,10 @@ static int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem, struct
>>   	}
>>   
>>   	if (obj->import_attach) {
>> -		ret = dma_buf_vmap(obj->import_attach->dmabuf, map);
>> +		if (local)
>> +			ret = dma_buf_vmap_local(obj->import_attach->dmabuf, map);
>> +		else
>> +			ret = dma_buf_vmap(obj->import_attach->dmabuf, map);
>>   		if (!ret) {
>>   			if (WARN_ON(map->is_iomem)) {
>>   				ret = -EIO;
>> @@ -313,7 +319,7 @@ static int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem, struct
>>   	return ret;
>>   }
>>   
>> -/*
>> +/**
>>    * drm_gem_shmem_vmap - Create a virtual mapping for a shmem GEM object
>>    * @shmem: shmem GEM object
>>    * @map: Returns the kernel virtual address of the SHMEM GEM object's backing
>> @@ -339,15 +345,53 @@ int drm_gem_shmem_vmap(struct drm_gem_object *obj, struct dma_buf_map *map)
>>   	ret = mutex_lock_interruptible(&shmem->vmap_lock);
>>   	if (ret)
>>   		return ret;
>> -	ret = drm_gem_shmem_vmap_locked(shmem, map);
>> +	ret = drm_gem_shmem_vmap_locked(shmem, map, false);
>>   	mutex_unlock(&shmem->vmap_lock);
>>   
>>   	return ret;
>>   }
>>   EXPORT_SYMBOL(drm_gem_shmem_vmap);
>>   
>> +/**
>> + * drm_gem_shmem_vmap_local - Create a virtual mapping for a shmem GEM object
>> + * @shmem: shmem GEM object
>> + * @map: Returns the kernel virtual address of the SHMEM GEM object's backing
>> + *       store.
>> + *
>> + * This function makes sure that a contiguous kernel virtual address mapping
>> + * exists for the buffer backing the shmem GEM object.
>> + *
>> + * The function is called with the BO's reservation object locked. Callers must
>> + * hold the lock until after unmapping the buffer.
>> + *
>> + * This function can be used to implement &drm_gem_object_funcs.vmap_local. But
>> + * it can also be called by drivers directly, in which case it will hide the
>> + * differences between dma-buf imported and natively allocated objects.
> 
> So for the other callbacks I tried to make sure we have different entry
> points for this, since it's not really the same thing and because of the
> locking mess we have with dma_resv_lock vs various pre-existing local
> locking scheme, it's easy to get a mess.
> 
> I think the super clean version here would be to also export just the
> internal stuff for the ->v(un)map_local hooks, but that's maybe a bit too
> much boilerplate for no real gain.
> -Daniel
> 
>> + *
>> + * Acquired mappings should be cleaned up by calling drm_gem_shmem_vunmap_local().
>> + *
>> + * Returns:
>> + * 0 on success or a negative error code on failure.
>> + */
>> +int drm_gem_shmem_vmap_local(struct drm_gem_object *obj, struct dma_buf_map *map)
>> +{
>> +	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
>> +	int ret;
>> +
>> +	dma_resv_assert_held(obj->resv);
>> +
>> +	ret = mutex_lock_interruptible(&shmem->vmap_lock);
>> +	if (ret)
>> +		return ret;
>> +	ret = drm_gem_shmem_vmap_locked(shmem, map, true);
>> +	mutex_unlock(&shmem->vmap_lock);
>> +
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL(drm_gem_shmem_vmap_local);
>> +
>>   static void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
>> -					struct dma_buf_map *map)
>> +					struct dma_buf_map *map, bool local)
>>   {
>>   	struct drm_gem_object *obj = &shmem->base;
>>   
>> @@ -358,7 +402,10 @@ static void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
>>   		return;
>>   
>>   	if (obj->import_attach)
>> -		dma_buf_vunmap(obj->import_attach->dmabuf, map);
>> +		if (local)
>> +			dma_buf_vunmap_local(obj->import_attach->dmabuf, map);
>> +		else
>> +			dma_buf_vunmap(obj->import_attach->dmabuf, map);
>>   	else
>>   		vunmap(shmem->vaddr);
>>   
>> @@ -366,7 +413,7 @@ static void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
>>   	drm_gem_shmem_put_pages(shmem);
>>   }
>>   
>> -/*
>> +/**
>>    * drm_gem_shmem_vunmap - Unmap a virtual mapping fo a shmem GEM object
>>    * @shmem: shmem GEM object
>>    * @map: Kernel virtual address where the SHMEM GEM object was mapped
>> @@ -384,11 +431,38 @@ void drm_gem_shmem_vunmap(struct drm_gem_object *obj, struct dma_buf_map *map)
>>   	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
>>   
>>   	mutex_lock(&shmem->vmap_lock);
>> -	drm_gem_shmem_vunmap_locked(shmem, map);
>> +	drm_gem_shmem_vunmap_locked(shmem, map, false);
>>   	mutex_unlock(&shmem->vmap_lock);
>>   }
>>   EXPORT_SYMBOL(drm_gem_shmem_vunmap);
>>   
>> +/**
>> + * drm_gem_shmem_vunmap_local - Unmap a virtual mapping fo a shmem GEM object
>> + * @shmem: shmem GEM object
>> + * @map: Kernel virtual address where the SHMEM GEM object was mapped
>> + *
>> + * This function cleans up a kernel virtual address mapping acquired by
>> + * drm_gem_shmem_vmap_local(). The mapping is only removed when the use count
>> + * drops to zero.
>> + *
>> + * The function is called with the BO's reservation object locked.
>> + *
>> + * This function can be used to implement &drm_gem_object_funcs.vmap_local.
>> + * But it can also be called by drivers directly, in which case it will hide
>> + * the differences between dma-buf imported and natively allocated objects.
>> + */
>> +void drm_gem_shmem_vunmap_local(struct drm_gem_object *obj, struct dma_buf_map *map)
>> +{
>> +	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
>> +
>> +	dma_resv_assert_held(obj->resv);
>> +
>> +	mutex_lock(&shmem->vmap_lock);
>> +	drm_gem_shmem_vunmap_locked(shmem, map, true);
>> +	mutex_unlock(&shmem->vmap_lock);
>> +}
>> +EXPORT_SYMBOL(drm_gem_shmem_vunmap_local);
>> +
>>   struct drm_gem_shmem_object *
>>   drm_gem_shmem_create_with_handle(struct drm_file *file_priv,
>>   				 struct drm_device *dev, size_t size,
>> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
>> index 434328d8a0d9..3f59bdf749aa 100644
>> --- a/include/drm/drm_gem_shmem_helper.h
>> +++ b/include/drm/drm_gem_shmem_helper.h
>> @@ -114,7 +114,9 @@ void drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem);
>>   int drm_gem_shmem_pin(struct drm_gem_object *obj);
>>   void drm_gem_shmem_unpin(struct drm_gem_object *obj);
>>   int drm_gem_shmem_vmap(struct drm_gem_object *obj, struct dma_buf_map *map);
>> +int drm_gem_shmem_vmap_local(struct drm_gem_object *obj, struct dma_buf_map *map);
>>   void drm_gem_shmem_vunmap(struct drm_gem_object *obj, struct dma_buf_map *map);
>> +void drm_gem_shmem_vunmap_local(struct drm_gem_object *obj, struct dma_buf_map *map);
>>   
>>   int drm_gem_shmem_madvise(struct drm_gem_object *obj, int madv);
>>   
>> -- 
>> 2.29.2
>>
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer


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