drm/i915: Document our internal limit on object size

In many places, we try to count pages using a 32 bit integer. That
implies if we are asked to create an object larger than 43bits, we will
subtly crash much later. Catch this on the boundary, and add a warning
to remind ourselves later on our exabyte systems.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161018120251.25043-2-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index cabcd06..33c44c6 100644 (file)
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -3107,7 +3107,7 @@ void i915_gem_object_free(struct drm_i915_gem_object *obj);
 void i915_gem_object_init(struct drm_i915_gem_object *obj,
                         const struct drm_i915_gem_object_ops *ops);
 struct drm_i915_gem_object *i915_gem_object_create(struct drm_device *dev,
-                                                 size_t size);
+                                                  u64 size);
 struct drm_i915_gem_object *i915_gem_object_create_from_data(
                struct drm_device *dev, const void *data, size_t size);
 void i915_gem_close_object(struct drm_gem_object *gem, struct drm_file *file);

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 838dc15..6165a3b 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4131,14 +4131,29 @@ static const struct drm_i915_gem_object_ops i915_gem_object_ops = {
        .put_pages = i915_gem_object_put_pages_gtt,
 };
 
-struct drm_i915_gem_object *i915_gem_object_create(struct drm_device *dev,
-                                                 size_t size)
+/* Note we don't consider signbits :| */
+#define overflows_type(x, T) \
+       (sizeof(x) > sizeof(T) && (x) >> (sizeof(T) * BITS_PER_BYTE))
+
+struct drm_i915_gem_object *
+i915_gem_object_create(struct drm_device *dev, u64 size)
 {
        struct drm_i915_gem_object *obj;
        struct address_space *mapping;
        gfp_t mask;
        int ret;
 
+       /* There is a prevalence of the assumption that we fit the object's
+        * page count inside a 32bit _signed_ variable. Let's document this and
+        * catch if we ever need to fix it. In the meantime, if you do spot
+        * such a local variable, please consider fixing!
+        */
+       if (WARN_ON(size >> PAGE_SHIFT > INT_MAX))
+               return ERR_PTR(-E2BIG);
+
+       if (overflows_type(size, obj->base.size))
+               return ERR_PTR(-E2BIG);
+
        obj = i915_gem_object_alloc(dev);
        if (obj == NULL)
                return ERR_PTR(-ENOMEM);