Re: [RFC PATCH 4/5] drm/ttm: Document and optimize ttm_bo_pipeline_gutting()

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On 5/21/21 10:21 AM, Christian König wrote:
Am 20.05.21 um 17:09 schrieb Thomas Hellström:
If the bo is idle when calling ttm_bo_pipeline_gutting(), we unnecessarily
create a ghost object and push it out to delayed destroy.
Fix this by adding a path for idle, and document the function.

Also avoid having the bo end up in a bad state vulnerable to user-space
triggered kernel BUGs if the call to ttm_tt_create() fails.

Finally reuse ttm_bo_pipeline_gutting() in ttm_bo_evict().

Cc: Christian König <christian.koenig@xxxxxxx>
Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
---
  drivers/gpu/drm/ttm/ttm_bo.c      | 20 +++++-----
  drivers/gpu/drm/ttm/ttm_bo_util.c | 63 ++++++++++++++++++++++++-------
  drivers/gpu/drm/ttm/ttm_tt.c      |  5 +++
  include/drm/ttm/ttm_tt.h          | 10 +++++
  4 files changed, 75 insertions(+), 23 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
index ca1b098b6a56..a8fa3375b8aa 100644
--- a/drivers/gpu/drm/ttm/ttm_bo.c
+++ b/drivers/gpu/drm/ttm/ttm_bo.c
@@ -501,10 +501,15 @@ static int ttm_bo_evict(struct ttm_buffer_object *bo,
      bdev->funcs->evict_flags(bo, &placement);
        if (!placement.num_placement && !placement.num_busy_placement) {
-        ttm_bo_wait(bo, false, false);
+        ret = ttm_bo_wait(bo, true, false);
+        if (ret)
+            return ret;
  -        ttm_bo_cleanup_memtype_use(bo);
-        return ttm_tt_create(bo, false);
+        /*
+         * Since we've already synced, this frees backing store
+         * immediately.
+         */
+        return ttm_bo_pipeline_gutting(bo);

Yeah, we tried to avoid pipeline_gutting here because of eviction. But I think when you wait before that should work.

      }
        ret = ttm_bo_mem_space(bo, &placement, &evict_mem, ctx);
@@ -974,13 +979,8 @@ int ttm_bo_validate(struct ttm_buffer_object *bo,
      /*
       * Remove the backing store if no placement is given.
       */
-    if (!placement->num_placement && !placement->num_busy_placement) {
-        ret = ttm_bo_pipeline_gutting(bo);
-        if (ret)
-            return ret;
-
-        return ttm_tt_create(bo, false);
-    }
+    if (!placement->num_placement && !placement->num_busy_placement)
+        return ttm_bo_pipeline_gutting(bo);
        /*
       * Check whether we need to move buffer.
diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
index 919ee03f7eb3..1860e2e7563f 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_util.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
@@ -479,7 +479,8 @@ static void ttm_transfered_destroy(struct ttm_buffer_object *bo)
   */
    static int ttm_buffer_object_transfer(struct ttm_buffer_object *bo,
-                      struct ttm_buffer_object **new_obj)
+                      struct ttm_buffer_object **new_obj,
+                      bool realloc_tt)
  {
      struct ttm_transfer_obj *fbo;
      int ret;
@@ -493,6 +494,17 @@ static int ttm_buffer_object_transfer(struct ttm_buffer_object *bo,
      ttm_bo_get(bo);
      fbo->bo = bo;
  +    if (realloc_tt) {
+        bo->ttm = NULL;
+        ret = ttm_tt_create(bo, true);
+        if (ret) {
+            bo->ttm = fbo->base.ttm;
+            kfree(fbo);
+            ttm_bo_put(bo);
+            return ret;
+        }
+    }
+

Can't we keep that logic in the caller? I think that would be cleaner.

Indeed, let me see if we can do that without breaking anything.


      /**
       * Fix up members that we shouldn't copy directly:
       * TODO: Explicit member copy would probably be better here.
@@ -763,7 +775,7 @@ static int ttm_bo_move_to_ghost(struct ttm_buffer_object *bo,
      dma_fence_put(bo->moving);
      bo->moving = dma_fence_get(fence);
  -    ret = ttm_buffer_object_transfer(bo, &ghost_obj);
+    ret = ttm_buffer_object_transfer(bo, &ghost_obj, false);
      if (ret)
          return ret;
  @@ -836,26 +848,51 @@ int ttm_bo_move_accel_cleanup(struct ttm_buffer_object *bo,
  }
  EXPORT_SYMBOL(ttm_bo_move_accel_cleanup);
  +/**
+ * ttm_bo_pipeline_gutting - purge the contents of a bo
+ * @bo: The buffer object
+ *
+ * Purge the contents of a bo, async if the bo is not idle.
+ * After a successful call, the bo is left unpopulated in
+ * system placement. The function may wait uninterruptible
+ * for idle on OOM.
+ *
+ * Return: 0 if successful, negative error code on failure.
+ */
  int ttm_bo_pipeline_gutting(struct ttm_buffer_object *bo)
  {
      static const struct ttm_place sys_mem = { .mem_type = TTM_PL_SYSTEM };
      struct ttm_buffer_object *ghost;
      int ret;
  -    ret = ttm_buffer_object_transfer(bo, &ghost);
-    if (ret)
-        return ret;
+    /* If already idle, no need for ghost object dance. */
+    ret = ttm_bo_wait(bo, false, true);
+    if (ret == -EBUSY) {
+        ret = ttm_buffer_object_transfer(bo, &ghost, true);
+        if (ret)
+            return ret;

When this is a shortcout to avoid work we should rather use the inverse notation.

In other words something like that:

if (ret != -EBUSY) {
    ttm_resource_free(bo, &bo->mem);
    ttm_resource_alloc(bo, &sys_mem, &bo->mem);
    ttm_tt_create()...
    return ret;
}

OK.


  -    ret = dma_resv_copy_fences(&ghost->base._resv, bo->base.resv);
-    /* Last resort, wait for the BO to be idle when we are OOM */
-    if (ret)
-        ttm_bo_wait(bo, false, false);
+        ret = dma_resv_copy_fences(&ghost->base._resv, bo->base.resv);
+        /* Last resort, wait for the BO to be idle when we are OOM */
+        if (ret)
+            ttm_bo_wait(bo, false, false);
  -    ttm_resource_alloc(bo, &sys_mem, &bo->mem);
-    bo->ttm = NULL;
+        dma_resv_unlock(&ghost->base._resv);
+        ttm_bo_put(ghost);
+    } else {
+        if (!bo->ttm) {
+            ret = ttm_tt_create(bo, true);
+            if (ret)
+                return ret;
+        } else {
+            ttm_tt_unpopulate(bo->bdev, bo->ttm);
+            if (bo->type == ttm_bo_type_device)
+                ttm_tt_mark_for_clear(bo->ttm);

That's not legal, you can't unpopulate it when the BO is busy.

Instead the TT object must be destroyed with the ghost and a new one created.

We've already verified that the bo is idle here, so we should be fine.

/Thomas



Christian.

+        }
+        ttm_resource_free(bo, &bo->mem);
+    }
  -    dma_resv_unlock(&ghost->base._resv);
-    ttm_bo_put(ghost);
+    ttm_resource_alloc(bo, &sys_mem, &bo->mem);
        return 0;
  }
diff --git a/drivers/gpu/drm/ttm/ttm_tt.c b/drivers/gpu/drm/ttm/ttm_tt.c
index 539e0232cb3b..0b1053e93db2 100644
--- a/drivers/gpu/drm/ttm/ttm_tt.c
+++ b/drivers/gpu/drm/ttm/ttm_tt.c
@@ -134,6 +134,11 @@ void ttm_tt_destroy_common(struct ttm_device *bdev, struct ttm_tt *ttm)
  }
  EXPORT_SYMBOL(ttm_tt_destroy_common);
  +void ttm_tt_mark_for_clear(struct ttm_tt *ttm)
+{
+    ttm->page_flags |= TTM_PAGE_FLAG_ZERO_ALLOC;
+}
+
  void ttm_tt_destroy(struct ttm_device *bdev, struct ttm_tt *ttm)
  {
      bdev->funcs->ttm_tt_destroy(bdev, ttm);
diff --git a/include/drm/ttm/ttm_tt.h b/include/drm/ttm/ttm_tt.h
index 134d09ef7766..91552c83ac79 100644
--- a/include/drm/ttm/ttm_tt.h
+++ b/include/drm/ttm/ttm_tt.h
@@ -157,6 +157,16 @@ int ttm_tt_populate(struct ttm_device *bdev, struct ttm_tt *ttm, struct ttm_oper
   */
  void ttm_tt_unpopulate(struct ttm_device *bdev, struct ttm_tt *ttm);
  +/**
+ * ttm_tt_mark_for_clear - Mark pages for clearing on populate.
+ *
+ * @ttm: Pointer to the ttm_tt structure
+ *
+ * Marks pages for clearing so that the next time the page vector is
+ * populated, the pages will be cleared.
+ */
+void ttm_tt_mark_for_clear(struct ttm_tt *ttm);
+
  void ttm_tt_mgr_init(unsigned long num_pages, unsigned long num_dma32_pages);
    #if IS_ENABLED(CONFIG_AGP)




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