Maintain pre-zygote fork linear-alloc pages as shared-clean

Userfaultfd tends to dirty all the pages of the space it is used on.
However, we want to maintain the shared-clean trait of the pages
allocated in zygote process prior to first fork.

This CL separates the pre-zygote fork arenas from the userfaultfd
visited ones, thereby reataining the former's shared-clean trait.

Bug: 160737021
Test: module install and enable uffd GC
Change-Id: Iddffb2c8d2d234ce7b20c069d86341dda5443a9b

1 files changed, 6 insertions, 0 deletions

diff --git a/runtime/linear_alloc-inl.h b/runtime/linear_alloc-inl.h
index 928bffbc1c..13dbea11d0 100644
--- a/runtime/linear_alloc-inl.h
+++ b/runtime/linear_alloc-inl.h
@@ -40,6 +40,12 @@ inline void LinearAlloc::SetFirstObject(void* begin, size_t bytes) const {
   down_cast<TrackedArena*>(arena)->SetFirstObject(static_cast<uint8_t*>(begin), end);
 }
 
+inline void LinearAlloc::SetupForPostZygoteFork(Thread* self) {
+  MutexLock mu(self, lock_);
+  DCHECK(track_allocations_);
+  allocator_.ResetCurrentArena();
+}
+
 inline void* LinearAlloc::Realloc(Thread* self,
                                   void* ptr,
                                   size_t old_size,