The swap file code does not go through the filesystem. It's the exact same code which is used for a swap partition or a swap file, and it writes directly to the

The swap file code does not go through the filesystem. It's the exact same code which is used for a swap partition or a swap file, and it writes directly to the block device (the partition). The main difference is that, if you have a swap file, the "swapon" call asks the filesystem to tell it which ranges in the block device contain the swap, instead of using the whole block device.