quoted

mm-page_file_methods.patch

    This makes page_offset and others more expensive by adding a
    conditional jump to a function call that is not usually made.

    Why do swap pages have a different index to everyone else?

Because the page->index of an anonymous page is related to its (anon)vma
so that it satisfies the constraints for vm_normal_page().

The index in the swap file it totally unrelated and quite random. Hence
the swap-cache uses page->private to store it in.

Yeah, and putting the condition into page_offset() will confuse code
which uses it for finding the offset in the VMA or in a tmpfs file.

So why not just have a separate page_swap_offset() function, used
exclusively by swap_in/out()?

Miklos