[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.osipe...@collabora.com>
---
 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