[PATCH 3/3] drm/ttm, drm/xe: Drop the ttm_tt on null moves to VRAM

[Matthew Auld <matthew.auld@intel.com>]

From: Thomas Hellström <thomas.hellstrom@linux.intel.com>

A BO created with DRM_XE_GEM_CREATE_FLAG_DEFER_BACKING starts in system
placement with an unpopulated ttm_tt. Its initial move to VRAM has no
source data and uses ttm_bo_move_null(), which unlike
ttm_bo_move_accel_cleanup() leaves bo->ttm attached. If the VRAM BO is
later exported, ttm_bo_setup_export() populates that stale ttm_tt,
pinning unused, unshrinkable host pages equal to the BO size and
potentially exhausting host RAM.

Add ttm_bo_move_null_drop_tt() to destroy the ttm_tt when moving to a
manager that doesn't use one, and use it in xe_bo_move().

Fixes: 91494dee1091 ("xe: populate buffers before exporting them.")
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
---
 drivers/gpu/drm/ttm/ttm_bo_util.c | 20 ++++++++++++++++++++
 drivers/gpu/drm/xe/xe_bo.c        |  6 +++---
 include/drm/ttm/ttm_bo.h          |  2 ++
 3 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
index f9458a4d5a09..0ab52be3a5f7 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_util.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
@@ -756,6 +756,26 @@ void ttm_bo_move_sync_cleanup(struct ttm_buffer_object *bo,
 }
 EXPORT_SYMBOL(ttm_bo_move_sync_cleanup);
 
+/**
+ * ttm_bo_move_null_drop_tt - assign memory without moving data, dropping the TT
+ * @bo: The buffer object.
+ * @new_mem: The memory to be assigned.
+ *
+ * Like ttm_bo_move_null(), but if the memory manager of @new_mem doesn't use
+ * a struct ttm_tt, also destroy @bo's ttm_tt, mimicking what
+ * ttm_bo_move_accel_cleanup() does. The caller must hold the bo's
+ * reservation, and the ttm_tt must not hold data that needs to be preserved.
+ */
+void ttm_bo_move_null_drop_tt(struct ttm_buffer_object *bo,
+			      struct ttm_resource *new_mem)
+{
+	if (!ttm_manager_type(bo->bdev, new_mem->mem_type)->use_tt)
+		ttm_bo_tt_destroy(bo);
+
+	ttm_bo_move_null(bo, new_mem);
+}
+EXPORT_SYMBOL(ttm_bo_move_null_drop_tt);
+
 /**
  * ttm_bo_pipeline_gutting - purge the contents of a bo
  * @bo: The buffer object
diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
index 0181f73ef66d..6628db44951f 100644
--- a/drivers/gpu/drm/xe/xe_bo.c
+++ b/drivers/gpu/drm/xe/xe_bo.c
@@ -1178,7 +1178,7 @@ static int xe_bo_move(struct ttm_buffer_object *ttm_bo, bool evict,
 	}
 
 	if ((move_lacks_source && !needs_clear)) {
-		ttm_bo_move_null(ttm_bo, new_mem);
+		ttm_bo_move_null_drop_tt(ttm_bo, new_mem);
 		goto out;
 	}
 
@@ -1313,7 +1313,7 @@ static int xe_bo_move(struct ttm_buffer_object *ttm_bo, bool evict,
 						new_mem);
 		if (ret) {
 			dma_fence_wait(fence, false);
-			ttm_bo_move_null(ttm_bo, new_mem);
+			ttm_bo_move_null_drop_tt(ttm_bo, new_mem);
 			ret = 0;
 		}
 	} else {
@@ -1324,7 +1324,7 @@ static int xe_bo_move(struct ttm_buffer_object *ttm_bo, bool evict,
 		 */
 		dma_resv_add_fence(ttm_bo->base.resv, fence,
 				   DMA_RESV_USAGE_KERNEL);
-		ttm_bo_move_null(ttm_bo, new_mem);
+		ttm_bo_move_null_drop_tt(ttm_bo, new_mem);
 	}
 
 	/*
diff --git a/include/drm/ttm/ttm_bo.h b/include/drm/ttm/ttm_bo.h
index 32791c4db2a9..123ef68bd849 100644
--- a/include/drm/ttm/ttm_bo.h
+++ b/include/drm/ttm/ttm_bo.h
@@ -480,6 +480,8 @@ int ttm_bo_move_accel_cleanup(struct ttm_buffer_object *bo,
 			      struct ttm_resource *new_mem);
 void ttm_bo_move_sync_cleanup(struct ttm_buffer_object *bo,
 			      struct ttm_resource *new_mem);
+void ttm_bo_move_null_drop_tt(struct ttm_buffer_object *bo,
+			      struct ttm_resource *new_mem);
 int ttm_bo_pipeline_gutting(struct ttm_buffer_object *bo);
 pgprot_t ttm_io_prot(struct ttm_buffer_object *bo, struct ttm_resource *res,
 		     pgprot_t tmp);

-- 
2.56.0