support O_NOATIME on linux and use it by default

include/libtorrent/file.hpp

@@ -177,6 +177,7 @@ namespace libtorrent
 			no_buffer = 4,
 			mode_mask = rw_mask | no_buffer,
 			sparse = 8,
+			no_atime = 16,
 
 			attribute_hidden = 0x1000,
 			attribute_executable = 0x2000,

include/libtorrent/session_settings.hpp

@@ -190,6 +190,7 @@ namespace libtorrent
 			, guided_read_cache(true)
 			, default_cache_min_age(1)
 			, num_optimistic_unchoke_slots(0)
+			, no_atime_storage(true)
 		{}
 
 		// this is the user agent that will be sent to the tracker
@@ -709,6 +710,10 @@ namespace libtorrent
 		// the global number of optimistic unchokes
 		// 0 means automatic
 		int num_optimistic_unchoke_slots;
+
+		// if set to true, files won't have their atime updated
+		// on disk reads. This works on linux
+		bool no_atime_storage;
 	};
 
 #ifndef TORRENT_DISABLE_DHT

src/file.cpp

@@ -769,8 +769,17 @@ namespace libtorrent
 		static const int no_buffer_flag[] = {0, 0};
 #endif
 
+#ifdef O_NOATIME
+		static const int no_atime_flag[] = {0, O_NOATIME};
+#endif
+
  		m_fd = ::open(convert_to_native(path).c_str()
- 			, mode_array[mode & rw_mask] | no_buffer_flag[(mode & no_buffer) >> 2], permissions);
+ 			, mode_array[mode & rw_mask]
+			| no_buffer_flag[(mode & no_buffer) >> 2]
+#ifdef O_NOATIME
+			| no_atime_flag[(mode & no_atime) >> 4]
+#endif
+			, permissions);
 
 #ifdef TORRENT_LINUX
 		// workaround for linux bug

src/session_impl.cpp

@@ -1021,6 +1021,7 @@ namespace aux {
 			|| m_settings.allow_reordered_disk_operations != s.allow_reordered_disk_operations
 			|| m_settings.file_pool_size != s.file_pool_size
 			|| m_settings.volatile_read_cache != s.volatile_read_cache
+			|| m_settings.no_atime_storage!= s.no_atime_storage
 			|| m_settings.low_prio_disk != s.low_prio_disk)
 			update_disk_io_thread = true;
 

src/storage.cpp

@@ -1304,6 +1304,7 @@ ret:
 			&& ((fe.offset + fe.file_base) & (m_page_size-1)) == 0))
 			mode |= file::no_buffer;
 		if (!m_allocate_files) mode |= file::sparse;
+		if (m_settings && settings().no_atime_storage) mode |= file::no_atime;
 
 		return m_pool.open_file(const_cast<storage*>(this), combine_path(m_save_path, fe.path), mode, ec);
 	}