Quoting Chris Wilson (2019-07-16 13:49:27)
Following a try_to_unmap() we may want to remove the userptr and so call put_pages(). However, try_to_unmap() acquires the page lock and so we must avoid recursively locking the pages ourselves -- which means that we cannot safely acquire the lock around set_page_dirty(). Since we can't be sure of the lock, we have to risk skip dirtying the page, or else risk calling set_page_dirty() without a lock and so risk fs corruption.
Reported-by: Lionel Landwerlin lionel.g.landwerlin@intel.com Fixes: cb6d7c7dc7ff ("drm/i915/userptr: Acquire the page lock around set_page_dirty()") Signed-off-by: Chris Wilson chris@chris-wilson.co.uk Cc: Lionel Landwerlin lionel.g.landwerlin@intel.com Cc: Tvrtko Ursulin tvrtko.ursulin@intel.com Cc: stable@vger.kernel.org
drivers/gpu/drm/i915/gem/i915_gem_userptr.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c index b9d2bb15e4a6..1ad2047a6dbd 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c +++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c @@ -672,7 +672,7 @@ i915_gem_userptr_put_pages(struct drm_i915_gem_object *obj, obj->mm.dirty = false; for_each_sgt_page(page, sgt_iter, pages) {
if (obj->mm.dirty)
if (obj->mm.dirty && trylock_page(page)) { /* * As this may not be anonymous memory (e.g. shmem) * but exist on a real mapping, we have to lock@@ -680,8 +680,20 @@ i915_gem_userptr_put_pages(struct drm_i915_gem_object *obj, * the page reference is not sufficient to * prevent the inode from being truncated. * Play safe and take the lock.
** However...!** The mmu-notifier can be invalidated for a* migrate_page, that is alreadying holding the lock* on the page. Such a try_to_unmap() will result* in us calling put_pages() and so recursively try* to lock the page. We avoid that deadlock with* a trylock_page() and in exchange we risk missing* some page dirtying. */
set_page_dirty_lock(page);
set_page_dirty(page);unlock_page(page);}
It really seems like we have no choice but to only call set_page_dirty() while under the page lock, and the only way we can guarantee that without recursion is with a trylock...