simplify posix MappedFile::Init and add useful logs

net/disk_cache/blockfile/mapped_file_posix.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/disk_cache/blockfile/mapped_file.h"
 
 #include <errno.h>
 #include <sys/mman.h>
 
 #include "base/files/file_path.h"
 #include "base/logging.h"
 #include "net/disk_cache/disk_cache.h"
 
 namespace disk_cache {
 
 void* MappedFile::Init(const base::FilePath& name, size_t size) {
   DCHECK(!init_);
   if (init_ || !File::Init(name))
     return NULL;
 
   size_t temp_len = size ? size : 4096;
   if (!size)
     size = GetLength();
 
   buffer_ = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED,
                  platform_file(), 0);
   init_ = true;
   view_size_ = size;
-  DCHECK(reinterpret_cast<intptr_t>(buffer_) != -1);
-  if (reinterpret_cast<intptr_t>(buffer_) == -1)
+  DPLOG_IF(FATAL, buffer_ == MAP_FAILED) << "Failed to mmap " << name.value();
+  if (buffer_ == MAP_FAILED)
     buffer_ = 0;
 
   // Make sure we detect hardware failures reading the headers.
   scoped_ptr<char[]> temp(new char[temp_len]);
   if (!Read(temp.get(), temp_len, 0))
     return NULL;
 
   return buffer_;
 }
 
 void MappedFile::Flush() {
 }
 
 MappedFile::~MappedFile() {
   if (!init_)
     return;
 
   if (buffer_) {
     int ret = munmap(buffer_, view_size_);
     DCHECK_EQ(0, ret);
   }
 }
 
 }  // namespace disk_cache