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drm/i915: Restrict pagefault disabling to just around copy_from_user()

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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@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161018120251.25043-4-chris@chris-wilson.co.uk
This commit is contained in:
Chris Wilson 2016-10-18 13:02:51 +01:00
parent 4ff340f061
commit ebc0808fa2
1 changed files with 35 additions and 36 deletions
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71
drivers/gpu/drm/i915/i915_gem_execbuffer.c
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@ -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;
}