Re: [PATCH v15 12/23] drm/shmem-helper: Add and use pages_pin_count

[Dmitry Osipenko]

On 8/28/23 14:46, Boris Brezillon wrote:
> On Sun, 27 Aug 2023 20:54:38 +0300
> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> 
>> Add separate pages_pin_count for tracking of whether drm-shmem pages are
>> moveable or not. With the addition of memory shrinker support to drm-shmem,
>> the pages_use_count will no longer determine whether pages are hard-pinned
>> in memory, but whether pages exit and are soft-pinned (and could be swapped
>> out). The pages_pin_count > 1 will hard-pin pages in memory.
>>
>> Suggested-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
>> ---
>>  drivers/gpu/drm/drm_gem_shmem_helper.c | 22 +++++++++++++++++-----
>>  include/drm/drm_gem_shmem_helper.h     | 10 ++++++++++
>>  2 files changed, 27 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> index d545d3d227d7..1a7e5c332fd8 100644
>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> @@ -234,14 +234,22 @@ static int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem)
>>  
>>  	dma_resv_assert_held(shmem->base.resv);
>>  
>> +	if (kref_get_unless_zero(&shmem->pages_pin_count))
>> +		return 0;
>> +
>>  	ret = drm_gem_shmem_get_pages_locked(shmem);
>> +	if (!ret)
>> +		kref_init(&shmem->pages_pin_count);
>>  
>>  	return ret;
>>  }
>>  
>> -static void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem)
>> +static void drm_gem_shmem_kref_unpin_pages(struct kref *kref)
>>  {
>> -	dma_resv_assert_held(shmem->base.resv);
>> +	struct drm_gem_shmem_object *shmem;
>> +
>> +	shmem = container_of(kref, struct drm_gem_shmem_object,
>> +			     pages_pin_count);
>>  
>>  	drm_gem_shmem_put_pages_locked(shmem);
>>  }
>> @@ -263,6 +271,9 @@ int drm_gem_shmem_pin(struct drm_gem_shmem_object *shmem)
>>  
>>  	drm_WARN_ON(obj->dev, obj->import_attach);
>>  
>> +	if (kref_get_unless_zero(&shmem->pages_pin_count))
>> +		return 0;
>> +
>>  	ret = dma_resv_lock_interruptible(shmem->base.resv, NULL);
>>  	if (ret)
>>  		return ret;
>> @@ -286,9 +297,10 @@ void drm_gem_shmem_unpin(struct drm_gem_shmem_object *shmem)
>>  
>>  	drm_WARN_ON(obj->dev, obj->import_attach);
>>  
>> -	dma_resv_lock(shmem->base.resv, NULL);
>> -	drm_gem_shmem_unpin_locked(shmem);
>> -	dma_resv_unlock(shmem->base.resv);
>> +	if (kref_put_dma_resv(&shmem->pages_pin_count,
>> +			      drm_gem_shmem_kref_unpin_pages,
>> +			      obj->resv, NULL))
>> +		dma_resv_unlock(obj->resv);
>>  }
>>  EXPORT_SYMBOL_GPL(drm_gem_shmem_unpin);
>>  
>> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
>> index ec2d8b24e3cf..afb7cd671e2a 100644
>> --- a/include/drm/drm_gem_shmem_helper.h
>> +++ b/include/drm/drm_gem_shmem_helper.h
>> @@ -39,6 +39,16 @@ struct drm_gem_shmem_object {
>>  	 */
>>  	unsigned int pages_use_count;
>>  
>> +	/**
>> +	 * @pages_pin_count:
>> +	 *
>> +	 * Reference count on the pinned pages table.
>> +	 * The pages allowed to be evicted and purged by memory
>> +	 * shrinker only when the count is zero, otherwise pages
>> +	 * are hard-pinned in memory.
>> +	 */
>> +	struct kref pages_pin_count;
> 
> I know it's tempting to use kref for the pages use/pin count, but I'm
> wondering if we wouldn't be better using a refcount_t, which provides
> overflow/underflow protection while still letting us control how we
> want to handle the locking for 0 <-> 1 transitions. By doing that, we
> avoid introducing core locking changes that might be more
> controversial/longer to get accepted. Besides, I suspect the resulting
> code (the one using a refcount_t) won't be more verbose/complicated (no
> release functions needed if you don't use kref_put(), which makes
> things closer to what we have right now).

Alright, let's try to use refcount_t since Christian also doesn't like kref

-- 
Best regards,
Dmitry