[PATCH 4/4] drm/i915: Restrict pagefault disabling to just around copy_from_user()

[Chris Wilson]

When handling execbuf relocations, we play a delicate dance with
pagefault. We first try to access the user pages underneath our
struct_mutex. However, if those pages were inside a GEM object, we may
trigger a pagefault and deadlock as i915_gem_fault() tries to
recursively acquire struct_mutex. Instead, we choose to disable
pagefaulting around the copy_from_user whilst inside the struct_mutex
and handle the EFAULT by falling back to a copy outside the
struct_mutex.

We however presumed that disabling pagefaults would be expensive. It is
just an operation on the local current task. Cheap enough that we can
restrict the disable/enable to the critical section around the copy, and
so avoid having to handle the atomic sections within the relocation
handling itself.

v2: Just illustrate the broken error handling rather than argue why it
is safer to ignore it, for now.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
---
 drivers/gpu/drm/i915/i915_gem_execbuffer.c | 71 +++++++++++++++---------------
 1 file changed, 35 insertions(+), 36 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 204536a3087e..e52affdcc125 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -551,20 +551,6 @@ relocate_entry(struct drm_i915_gem_object *obj,
 	return 0;
 }
 
-static bool object_is_idle(struct drm_i915_gem_object *obj)
-{
-	unsigned long active = i915_gem_object_get_active(obj);
-	int idx;
-
-	for_each_active(active, idx) {
-		if (!i915_gem_active_is_idle(&obj->last_read[idx],
-					     &obj->base.dev->struct_mutex))
-			return false;
-	}
-
-	return true;
-}
-
 static int
 i915_gem_execbuffer_relocate_entry(struct drm_i915_gem_object *obj,
 				   struct eb_vmas *eb,
@@ -648,10 +634,6 @@ i915_gem_execbuffer_relocate_entry(struct drm_i915_gem_object *obj,
 		return -EINVAL;
 	}
 
-	/* We can't wait for rendering with pagefaults disabled */
-	if (pagefault_disabled() && !object_is_idle(obj))
-		return -EFAULT;
-
 	ret = relocate_entry(obj, reloc, cache, target_offset);
 	if (ret)
 		return ret;
@@ -678,12 +660,23 @@ i915_gem_execbuffer_relocate_vma(struct i915_vma *vma,
 	remain = entry->relocation_count;
 	while (remain) {
 		struct drm_i915_gem_relocation_entry *r = stack_reloc;
-		int count = remain;
-		if (count > ARRAY_SIZE(stack_reloc))
-			count = ARRAY_SIZE(stack_reloc);
+		unsigned long unwritten;
+		unsigned int count;
+
+		count = min_t(unsigned int, remain, ARRAY_SIZE(stack_reloc));
 		remain -= count;
 
-		if (__copy_from_user_inatomic(r, user_relocs, count*sizeof(r[0]))) {
+		/* This is the fast path and we cannot handle a pagefault
+		 * whilst holding the struct mutex lest the user pass in the
+		 * relocations contained within a mmaped bo. For in such a case
+		 * we, the page fault handler would call i915_gem_fault() and
+		 * we would try to acquire the struct mutex again. Obviously
+		 * this is bad and so lockdep complains vehemently.
+		 */
+		pagefault_disable();
+		unwritten = __copy_from_user_inatomic(r, user_relocs, count*sizeof(r[0]));
+		pagefault_enable();
+		if (unlikely(unwritten)) {
 			ret = -EFAULT;
 			goto out;
 		}
@@ -695,11 +688,26 @@ i915_gem_execbuffer_relocate_vma(struct i915_vma *vma,
 			if (ret)
 				goto out;
 
-			if (r->presumed_offset != offset &&
-			    __put_user(r->presumed_offset,
-				       &user_relocs->presumed_offset)) {
-				ret = -EFAULT;
-				goto out;
+			if (r->presumed_offset != offset) {
+				pagefault_disable();
+				unwritten = __put_user(r->presumed_offset,
+						       &user_relocs->presumed_offset);
+				pagefault_enable();
+				if (unlikely(unwritten)) {
+					/* Note that reporting an error now
+					 * leaves everything in an inconsistent
+					 * state as we have *already* changed
+					 * the relocation value inside the
+					 * object. As we have not changed the
+					 * reloc.presumed_offset or will not
+					 * change the execobject.offset, on the
+					 * call we may not rewrite the value
+					 * inside the object, leaving it
+					 * dangling and causing a GPU hang.
+					 */
+					ret = -EFAULT;
+					goto out;
+				}
 			}
 
 			user_relocs++;
@@ -739,20 +747,11 @@ i915_gem_execbuffer_relocate(struct eb_vmas *eb)
 	struct i915_vma *vma;
 	int ret = 0;
 
-	/* This is the fast path and we cannot handle a pagefault whilst
-	 * holding the struct mutex lest the user pass in the relocations
-	 * contained within a mmaped bo. For in such a case we, the page
-	 * fault handler would call i915_gem_fault() and we would try to
-	 * acquire the struct mutex again. Obviously this is bad and so
-	 * lockdep complains vehemently.
-	 */
-	pagefault_disable();
 	list_for_each_entry(vma, &eb->vmas, exec_list) {
 		ret = i915_gem_execbuffer_relocate_vma(vma, eb);
 		if (ret)
 			break;
 	}
-	pagefault_enable();
 
 	return ret;
 }
-- 
2.9.3