PCScan: Avoid purging PA memory if PCScan task is scheduled

This fixes the race between PCScan and memory reclaimer (if reclaimer
decommits a page that was snapshotted before running a PCScan task, the
scanner will hit a page fault).

Bug: 1156011
Change-Id: I90a9655962f4cc0849165287b3c8199d7401d386
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2579911Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835117}

base/allocator/partition_allocator/partition_root.cc

@@ -614,6 +614,12 @@ void PartitionRoot<thread_safe>::PurgeMemory(int flags) {
 
   {
     ScopedGuard guard{lock_};
+    // Avoid purging if there is PCScan task currently scheduled. Since pcscan
+    // takes snapshot of all allocated pages, decommitting pages here (even
+    // under the lock) is racy.
+    // TODO(bikineev): Consider rescheduling the purging after PCScan.
+    if (PCScan::Instance().IsInProgress())
+      return;
     if (flags & PartitionPurgeDecommitEmptySlotSpans)
       DecommitEmptySlotSpans();
     if (flags & PartitionPurgeDiscardUnusedSystemPages) {

base/allocator/partition_allocator/pcscan.h

@@ -73,6 +73,10 @@ class BASE_EXPORT PCScan final {
 
   void ClearRootsForTesting();
 
+  bool IsInProgress() const {
+    return in_progress_.load(std::memory_order_relaxed);
+  }
+
  private:
   class PCScanTask;
   class PCScanThread;