drm/i915: Ignore duplicate VMA stored within the per-object handle LUT

By using drm_gem_flink/drm_gem_open on an object using the same fd, it
is possible for a client to create multiple handles pointing to the same
object (tied to the same contexts and VMA), as exemplified by
igt::gem_handle_to_libdrm_bo(). Since this duplication has been possible
since forever, we cannot assume that the handle:(fpriv, object) is
unique and so must handle the multiple users of a single VMA.

v2: Added commentary noise.

Testcase: igt/gem_close
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102355
Fixes: d1b48c1e7184 ("drm/i915: Replace execbuf vma ht with an idr")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170822110517.22277-3-chris@chris-wilson.co.uk
Tested-by: Marta Lofstedt <marta.lofstedt@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
(cherry-picked from commit 3ffff01749928ea5ffdae2cecad561898c3b0f71)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -3262,7 +3262,13 @@ void i915_gem_close_object(struct drm_gem_object *gem, struct drm_file *file)
 
 		vma = radix_tree_delete(&ctx->handles_vma, lut->handle);
 
-		if (!i915_vma_is_ggtt(vma))
+		GEM_BUG_ON(vma->obj != obj);
+
+		/* We allow the process to have multiple handles to the same
+		 * vma, in the same fd namespace, by virtue of flink/open.
+		 */
+		GEM_BUG_ON(!vma->open_count);
+		if (!--vma->open_count && !i915_vma_is_ggtt(vma))
 			i915_vma_close(vma);
 
 		list_del(&lut->obj_link);

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -720,6 +720,7 @@ static int eb_lookup_vmas(struct i915_execbuffer *eb)
 			goto err_obj;
 		}
 
+		vma->open_count++;
 		list_add(&lut->obj_link, &obj->lut_list);
 		list_add(&lut->ctx_link, &eb->ctx->handles_list);
 		lut->ctx = eb->ctx;

drivers/gpu/drm/i915/i915_vma.h

@@ -59,6 +59,12 @@ struct i915_vma {
 	u32 fence_size;
 	u32 fence_alignment;
 
+	/**
+	 * Count of the number of times this vma has been opened by different
+	 * handles (but same file) for execbuf, i.e. the number of aliases
+	 * that exist in the ctx->handle_vmas LUT for this vma.
+	 */
+	unsigned int open_count;
 	unsigned int flags;
 	/**
 	 * How many users have pinned this object in GTT space. The following