Add guard pages in front of platform allocations

src/spaces.cc

Index: src/spaces.cc
===================================================================
--- src/spaces.cc	(revision 8657)
+++ src/spaces.cc	(working copy)
@@ -407,7 +407,8 @@
   if (isolate_->code_range()->contains(static_cast<Address>(mem))) {
     isolate_->code_range()->FreeRawMemory(mem, length);
   } else {
-    OS::Free(mem, length);
+    size_t guardsize = (executable == EXECUTABLE) ? Page::kPageSize : 0;
+    OS::Free(static_cast<char*>(mem) - guardsize, length + guardsize);

[Mads Ager (chromium), 2011/07/17 09:47:53]

This looks nasty to me. Doesn't this mean that to free executable memory you
have to pass in another address than the one you got from AllocateRawMemory? We
should avoid that.

[Cris Neckar, 2011/07/18 23:55:12]

Done.

   }
   isolate_->counters()->memory_allocated()->Decrement(static_cast<int>(length));
   size_ -= static_cast<int>(length);
@@ -503,6 +504,11 @@
   if (chunk == NULL) return Page::FromAddress(NULL);
   LOG(isolate_, NewEvent("PagedChunk", chunk, chunk_size));
 
+  if (owner->executable() == EXECUTABLE) {
+    OS::Guard(chunk, Page::kPageSize);
+    chunk_size -= Page::kPageSize;
+    chunk = static_cast<Address>(chunk) + Page::kPageSize;
+  }
   *allocated_pages = PagesInChunk(static_cast<Address>(chunk), chunk_size);
   // We may 'lose' a page due to alignment.
   ASSERT(*allocated_pages >= kPagesPerChunk - 1);
@@ -2672,9 +2678,10 @@
                                         Executability executable) {
   size_t requested = ChunkSizeFor(size_in_bytes);
   size_t size;
+  size_t guardsize = (executable == EXECUTABLE) ? Page::kPageSize : 0;
   Isolate* isolate = Isolate::Current();
   void* mem = isolate->memory_allocator()->AllocateRawMemory(
-      requested, &size, executable);
+      requested + guardsize, &size, executable);
   if (mem == NULL) return NULL;
 
   // The start of the chunk may be overlayed with a page so we have to
@@ -2684,11 +2691,17 @@
   LOG(isolate, NewEvent("LargeObjectChunk", mem, size));
   if (size < requested) {

[Mads Ager (chromium), 2011/07/17 09:47:53]

Shouldn't you add guardsize to requested before you get here? Otherwise you
might not have enough space for the guard page.

[Cris Neckar, 2011/07/18 23:55:12]

Done.

     isolate->memory_allocator()->FreeRawMemory(
-        mem, size, executable);
+        static_cast<Address>(mem) - guardsize, size + guardsize, executable);

[Mads Ager (chromium), 2011/07/17 09:47:53]

I'm getting confused here. Does this add up? FreeRawMemory will subtract
guardsize again, right?

[Cris Neckar, 2011/07/18 23:55:12]

Yeah this was a mistake, I had initially planned to do it the way that you asked
and forgot to get rid of this.

Done.

     LOG(isolate, DeleteEvent("LargeObjectChunk", mem));
     return NULL;
   }
 
+  if (guardsize != 0) {
+    OS::Guard(mem, guardsize);
+    size -= guardsize;
+    mem = static_cast<Address>(mem) + guardsize;
+  }
+
   ObjectSpace space = (executable == EXECUTABLE)
       ? kObjectSpaceCodeSpace
       : kObjectSpaceLoSpace;