linux-kernel - Re: [PATCH v13 03/10] drm/shmem-helper: Add pages_pin

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Message-ID: <76624af3-1d82-8f96-6681-76538026395e@collabora.com>
Date:   Mon, 26 Jun 2023 18:32:16 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Boris Brezillon <boris.brezillon@...labora.com>
Cc:     David Airlie <airlied@...il.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Daniel Almeida <daniel.almeida@...labora.com>,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        Daniel Stone <daniel@...ishbar.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Qiang Yu <yuq825@...il.com>,
        Steven Price <steven.price@....com>,
        Rob Herring <robh@...nel.org>, intel-gfx@...ts.freedesktop.org,
        kernel@...labora.com, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v13 03/10] drm/shmem-helper: Add pages_pin_count field

On 6/26/23 18:21, Boris Brezillon wrote:
> On Mon, 26 Jun 2023 17:04:57 +0200
> Boris Brezillon <boris.brezillon@...labora.com> wrote:
> 
>> Hi Dmitry,
>>
>> Sorry for chiming in only now :-/.
>>
>> On Tue, 14 Mar 2023 05:26:52 +0300
>> Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:
>>
>>> And new pages_pin_count field to struct drm_gem_shmem_object that will
>>> determine whether pages are evictable by memory shrinker. The pages will
>>> be evictable only when pages_pin_count=0. This patch prepares code for
>>> addition of the memory shrinker that will utilize the new field.
>>>
>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
>>> ---
>>>  drivers/gpu/drm/drm_gem_shmem_helper.c | 7 +++++++
>>>  include/drm/drm_gem_shmem_helper.h     | 9 +++++++++
>>>  2 files changed, 16 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>> index 4da9c9c39b9a..81d61791f874 100644
>>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>> @@ -277,6 +277,8 @@ static int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem)
>>>  	drm_WARN_ON(obj->dev, obj->import_attach);
>>>  
>>>  	ret = drm_gem_shmem_get_pages(shmem);
>>> +	if (!ret)
>>> +		shmem->pages_pin_count++;
>>>  
>>>  	return ret;
>>>  }
>>> @@ -289,7 +291,12 @@ static void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem)
>>>  
>>>  	drm_WARN_ON(obj->dev, obj->import_attach);
>>>  
>>> +	if (drm_WARN_ON_ONCE(obj->dev, !shmem->pages_pin_count))
>>> +		return;
>>> +
>>>  	drm_gem_shmem_put_pages(shmem);
>>> +
>>> +	shmem->pages_pin_count--;
>>>  }
>>>  
>>>  /**
>>> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
>>> index 20ddcd799df9..7d823c9fc480 100644
>>> --- a/include/drm/drm_gem_shmem_helper.h
>>> +++ b/include/drm/drm_gem_shmem_helper.h
>>> @@ -39,6 +39,15 @@ 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 by memory shrinker
>>> +	 * only when the count is zero.
>>> +	 */
>>> +	unsigned int pages_pin_count;  
>>
>> s/pages_pin_count/pin_count/ ?
>>
>> And do we really need both pages_pin_count and pages_use_count. Looks
>> like they both serve the same purpose, with one exception:
>> pages_use_count is also incremented in the get_pages_sgt_locked() path,
>> but you probably don't want it to prevent GEM eviction. Assuming
>> your goal with this pin_count field is to check if a GEM object is
>> evictable, it can be done with something like
>>
>> bool
>> drm_gem_shmem_is_evictable_locked(struct drm_gem_shmem_object *shmem)
>> {
>> 	dma_resv_assert_held(shmem->base.resv);
>>
>> 	return shmem->pages_use_count == (shmem->sgt ? 1 : 0);
>> }
>>
>> I mean, I'm not against renaming pages_use_count into pin_count, but,
>> unless I'm missing something, I don't see a good reason to keep both.
> 
> My bad, I think I found one place calling drm_gem_shmem_get_pages()
> where we want pin_count and pages_use_count to differ:
> drm_gem_shmem_mmap(). We certainly don't want userspace mappings to
> prevent eviction.

That's correct, thanks for the review :)

-- 
Best regards,
Dmitry