More robust when allocation fails during compaction

R=ulan@chromium.org
BUG=chromium:473307
LOG=yes

Review URL: https://codereview.chromium.org/1068833002

Cr-Commit-Position: refs/heads/master@{#27637}

src/heap/mark-compact.cc

@@ -3322,12 +3322,13 @@ void MarkCompactCollector::EvacuatePages() {
     // Allocate emergency memory for the case when compaction fails due to out
     // of memory.
     if (!space->HasEmergencyMemory()) {
-      space->CreateEmergencyMemory();
+      space->CreateEmergencyMemory();  // If the OS lets us.
     }
     if (p->IsEvacuationCandidate()) {
-      // During compaction we might have to request a new page. Check that we
-      // have an emergency page and the space still has room for that.
-      if (space->HasEmergencyMemory() || space->CanExpand()) {
+      // During compaction we might have to request a new page in order to free
+      // up a page.  Check that we actually got an emergency page above so we
+      // can guarantee that this succeeds.
+      if (space->HasEmergencyMemory()) {
         EvacuateLiveObjectsFromPage(p);
         // Unlink the page from the list of pages here. We must not iterate
         // over that page later (e.g. when scan on scavenge pages are

src/heap/spaces.cc

@@ -1152,6 +1152,11 @@ void PagedSpace::FreeEmergencyMemory() {
 
 
 void PagedSpace::UseEmergencyMemory() {
+  // Page::Initialize makes the chunk into a real page and adds it to the
+  // accounting for this space.  Unlike PagedSpace::Expand, we don't check
+  // CanExpand first, so we can go over the limits a little here.  That's OK,
+  // because we are in the process of compacting which will free up at least as
+  // much memory as it allocates.
   Page* page = Page::Initialize(heap(), emergency_memory_, executable(), this);
   page->InsertAfter(anchor_.prev_page());
   emergency_memory_ = NULL;