base: Add free list implementation to browser-wide discardable memory system.

base/memory/discardable_memory_shmem.cc

Index: base/memory/discardable_memory_shmem.cc
diff --git a/base/memory/discardable_memory_shmem.cc b/base/memory/discardable_memory_shmem.cc
index 42824813f8452bb51462d311d932ac49cd47fc48..416fcf2aa9fede8388c4895ec4084307d84c75ae 100644
--- a/base/memory/discardable_memory_shmem.cc
+++ b/base/memory/discardable_memory_shmem.cc
@@ -88,33 +88,31 @@ void DiscardableMemoryShmem::Unlock() {
 
 void* DiscardableMemoryShmem::Memory() const {
   DCHECK(is_locked_);
-  DCHECK(shared_memory_);
-  return shared_memory_->memory();
+  DCHECK(chunk_);
+  return chunk_->Memory();
 }
 
 bool DiscardableMemoryShmem::AllocateAndAcquireLock() {
-  if (shared_memory_ && shared_memory_->Lock(0, 0))
+  if (chunk_ && chunk_->Lock())
     return true;
 
-  // TODO(reveman): Allocate fixed size memory segments and use a free list to
-  // improve performance and limit the number of file descriptors used.
-  shared_memory_ = DiscardableMemoryShmemAllocator::GetInstance()
-                       ->AllocateLockedDiscardableSharedMemory(bytes_);
-  DCHECK(shared_memory_);
+  chunk_ = DiscardableMemoryShmemAllocator::GetInstance()
+               ->AllocateLockedDiscardableMemory(bytes_);
+  DCHECK(chunk_);
   return false;
 }
 
 void DiscardableMemoryShmem::ReleaseLock() {
-  shared_memory_->Unlock(0, 0);
+  chunk_->Unlock();
 }
 
 void DiscardableMemoryShmem::Purge() {
-  shared_memory_->Purge(Time());
-  shared_memory_.reset();
+  DCHECK(!is_locked_);

[danakj, 2014/12/18 21:40:50]

Why no chunk_->Purge()?

[reveman, 2014/12/19 00:49:54]

The underlying shared memory can't be purged as it might be used by another
chunk. Deleting the chunk instance results in the behavior expected by the user
of this API. The memory is returned to the free list and a new chunk needs to be
allocated next time AllocateAndAcquireLock() is called.

Note: ::Purge() API will be removed when the Chunk class is removed.

+  chunk_.reset();
 }
 
 bool DiscardableMemoryShmem::IsMemoryResident() const {
-  return shared_memory_->IsMemoryResident();
+  return chunk_->IsMemoryResident();
 }
 
 }  // namespace internal