The fix is about properly handling the swapfile read/write offset. Axboe noted in the patch, "We're not factoring in the start of the file for where to write and read the swapfile, which leads to very unfortunate side effects of writing where we should not be..." In other words, clobbering the underlying file-system where the system's swapfile is placed.
I cannot stress the importance of such swapfile tests enough.
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