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Date:   Mon, 26 Jun 2023 17:21:48 +0200
From:   Boris Brezillon <boris.brezillon@collabora.com>
To:     Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc:     David Airlie <airlied@gmail.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Gurchetan Singh <gurchetansingh@chromium.org>,
        Chia-I Wu <olvaffe@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
        Daniel Almeida <daniel.almeida@collabora.com>,
        Gustavo Padovan <gustavo.padovan@collabora.com>,
        Daniel Stone <daniel@fooishbar.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Christian =?UTF-8?B?S8O2bmln?= <christian.koenig@amd.com>,
        Qiang Yu <yuq825@gmail.com>,
        Steven Price <steven.price@arm.com>,
        Rob Herring <robh@kernel.org>, intel-gfx@lists.freedesktop.org,
        kernel@collabora.com, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v13 03/10] drm/shmem-helper: Add pages_pin_count field
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On Mon, 26 Jun 2023 17:04:57 +0200
Boris Brezillon <boris.brezillon@collabora.com> wrote:

> Hi Dmitry,
> 
> Sorry for chiming in only now :-/.
> 
> On Tue, 14 Mar 2023 05:26:52 +0300
> Dmitry Osipenko <dmitry.osipenko@collabora.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@collabora.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.