Re: [PATCH v2 1/8] drm: Add dummy page per device or GEM object

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On 6/22/20 5:35 AM, Daniel Vetter wrote:
On Sun, Jun 21, 2020 at 02:03:01AM -0400, Andrey Grodzovsky wrote:
Will be used to reroute CPU mapped BO's page faults once
device is removed.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@xxxxxxx>
---
  drivers/gpu/drm/drm_file.c  |  8 ++++++++
  drivers/gpu/drm/drm_prime.c | 10 ++++++++++
  include/drm/drm_file.h      |  2 ++
  include/drm/drm_gem.h       |  2 ++
  4 files changed, 22 insertions(+)

diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
index c4c704e..67c0770 100644
--- a/drivers/gpu/drm/drm_file.c
+++ b/drivers/gpu/drm/drm_file.c
@@ -188,6 +188,12 @@ struct drm_file *drm_file_alloc(struct drm_minor *minor)
  			goto out_prime_destroy;
  	}
+ file->dummy_page = alloc_page(GFP_KERNEL | __GFP_ZERO);
+	if (!file->dummy_page) {
+		ret = -ENOMEM;
+		goto out_prime_destroy;
+	}
+
  	return file;
out_prime_destroy:
@@ -284,6 +290,8 @@ void drm_file_free(struct drm_file *file)
  	if (dev->driver->postclose)
  		dev->driver->postclose(dev, file);
+ __free_page(file->dummy_page);
+
  	drm_prime_destroy_file_private(&file->prime);
WARN_ON(!list_empty(&file->event_list));
diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 1de2cde..c482e9c 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -335,6 +335,13 @@ int drm_gem_prime_fd_to_handle(struct drm_device *dev,
ret = drm_prime_add_buf_handle(&file_priv->prime,
  			dma_buf, *handle);
+
+	if (!ret) {
+		obj->dummy_page = alloc_page(GFP_KERNEL | __GFP_ZERO);
+		if (!obj->dummy_page)
+			ret = -ENOMEM;
+	}
+
  	mutex_unlock(&file_priv->prime.lock);
  	if (ret)
  		goto fail;
@@ -1006,6 +1013,9 @@ void drm_prime_gem_destroy(struct drm_gem_object *obj, struct sg_table *sg)
  		dma_buf_unmap_attachment(attach, sg, DMA_BIDIRECTIONAL);
  	dma_buf = attach->dmabuf;
  	dma_buf_detach(attach->dmabuf, attach);
+
+	__free_page(obj->dummy_page);
+
  	/* remove the reference */
  	dma_buf_put(dma_buf);
  }
diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
index 19df802..349a658 100644
--- a/include/drm/drm_file.h
+++ b/include/drm/drm_file.h
@@ -335,6 +335,8 @@ struct drm_file {
  	 */
  	struct drm_prime_file_private prime;
Kerneldoc for these please, including why we need them and when. E.g. the
one in gem_bo should say it's only for exported buffers, so that we're not
colliding security spaces.

+	struct page *dummy_page;
+
  	/* private: */
  #if IS_ENABLED(CONFIG_DRM_LEGACY)
  	unsigned long lock_count; /* DRI1 legacy lock count */
diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index 0b37506..47460d1 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -310,6 +310,8 @@ struct drm_gem_object {
  	 *
  	 */
  	const struct drm_gem_object_funcs *funcs;
+
+	struct page *dummy_page;
  };
I think amdgpu doesn't care, but everyone else still might care somewhat
about flink. That also shares buffers, so also needs to allocate the
per-bo dummy page.


Hi, back to this topic after a long context switch for internal project.

I don't see why for FLINK we can't use same dummy page from struct drm_gem_object - looking at drm_gem_flink_ioctl I see that the underlying object we look up is still of type drm_gem_object.
Why we need per BO (TTM BO  I assume?) dummy page for this ?

Andrey



I also wonder whether we shouldn't have a helper to look up the dummy
page, just to encode in core code how it's supposedo to cascade.
-Daniel

/**
--
2.7.4

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