[RFC PATCH 2/5] drm/gem: track mm struct of allocating process in gem object

[Lucas Stach]

This keeps around a weak reference to the struct mm of the process
allocating the GEM object. This allows us to charge/uncharge the
process with the allocated backing store memory, even if this is
happening from another context.

Signed-off-by: Lucas Stach <l.stach at pengutronix.de>
---
 drivers/gpu/drm/drm_gem.c |  5 +++++
 include/drm/drm_gem.h     | 12 ++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 86d670c71286..b882f935cd4b 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -36,6 +36,7 @@
 #include <linux/pagemap.h>
 #include <linux/pagevec.h>
 #include <linux/shmem_fs.h>
+#include <linux/sched/mm.h>
 #include <linux/slab.h>
 #include <linux/string_helpers.h>
 #include <linux/types.h>
@@ -157,6 +158,9 @@ void drm_gem_private_object_init(struct drm_device *dev,
 	obj->dev = dev;
 	obj->filp = NULL;
 
+	mmgrab(current->mm);
+	obj->mm = current->mm;
+
 	kref_init(&obj->refcount);
 	obj->handle_count = 0;
 	obj->size = size;
@@ -949,6 +953,7 @@ drm_gem_object_release(struct drm_gem_object *obj)
 	if (obj->filp)
 		fput(obj->filp);
 
+	mmdrop(obj->mm);
 	dma_resv_fini(&obj->_resv);
 	drm_gem_free_mmap_offset(obj);
 }
diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index 87cffc9efa85..d021a083c282 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -234,6 +234,18 @@ struct drm_gem_object {
 	 */
 	struct drm_vma_offset_node vma_node;
 
+	/**
+	 * @mm:
+	 *
+	 * mm struct of the process creating the object. Used to account the
+	 * allocated backing store memory.
+	 *
+	 * Note that this is a weak reference created by mmgrab(), so any
+	 * manipulation needs to make sure the address space is still around by
+	 * calling mmget_not_zero().
+	 */
+	struct mm_struct *mm;
+
 	/**
 	 * @size:
 	 *
-- 
2.30.2