[PATCH v16 02/20] drm/shmem-helper: Use flag for tracking page count bumped by get_pages_sgt()

[Dmitry Osipenko]

Use separate flag for tracking page count bumped by shmem->sgt to avoid
imbalanced page counter during of drm_gem_shmem_free() time. It's fragile
to assume that populated shmem->pages at a freeing time means that the
count was bumped by drm_gem_shmem_get_pages_sgt(), using a flag removes
the ambiguity.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 11 ++++++++++-
 drivers/gpu/drm/lima/lima_gem.c        |  1 +
 include/drm/drm_gem_shmem_helper.h     |  7 +++++++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 6693d4061ca1..848435e08eb2 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -152,8 +152,10 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
 			sg_free_table(shmem->sgt);
 			kfree(shmem->sgt);
 		}
-		if (shmem->pages)
+		if (shmem->pages) {
 			drm_gem_shmem_put_pages(shmem);
+			drm_WARN_ON(obj->dev, !shmem->got_pages_sgt);
+		}
 
 		drm_WARN_ON(obj->dev, shmem->pages_use_count);
 
@@ -693,6 +695,13 @@ static struct sg_table *drm_gem_shmem_get_pages_sgt_locked(struct drm_gem_shmem_
 	if (ret)
 		goto err_free_sgt;
 
+	/*
+	 * This flag prevents imbalanced pages_use_count during
+	 * drm_gem_shmem_free(), where pages_use_count=1 only if
+	 * drm_gem_shmem_get_pages_sgt() was used by a driver.
+	 */
+	shmem->got_pages_sgt = true;
+
 	shmem->sgt = sgt;
 
 	return sgt;
diff --git a/drivers/gpu/drm/lima/lima_gem.c b/drivers/gpu/drm/lima/lima_gem.c
index 4f9736e5f929..67c39b95e30e 100644
--- a/drivers/gpu/drm/lima/lima_gem.c
+++ b/drivers/gpu/drm/lima/lima_gem.c
@@ -48,6 +48,7 @@ int lima_heap_alloc(struct lima_bo *bo, struct lima_vm *vm)
 
 		bo->base.pages = pages;
 		bo->base.pages_use_count = 1;
+		bo->base.got_pages_sgt = true;
 
 		mapping_set_unevictable(mapping);
 	}
diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
index ec70a98a8fe1..a53c0874b3c4 100644
--- a/include/drm/drm_gem_shmem_helper.h
+++ b/include/drm/drm_gem_shmem_helper.h
@@ -73,6 +73,13 @@ struct drm_gem_shmem_object {
 	 */
 	unsigned int vmap_use_count;
 
+	/**
+	 * @got_pages_sgt:
+	 *
+	 * True if SG table was retrieved using drm_gem_shmem_get_pages_sgt()
+	 */
+	bool got_pages_sgt : 1;
+
 	/**
 	 * @imported_sgt:
 	 *
-- 
2.41.0