Sorry, but are you running a 32 bit userland with your 64 bit kernel? Ifyou are, then the compatibility layer is involved, and sendfile is still taking 32 bit values...
I think I see a kernel bug here that would cause any count > (2 ** 31 - 1) to get garbled, but only with 32-bit compat mode and only on x86_64 (not ia64, the code there seems sane).
I'll read the code some more to make sure I'm not imagining things (I don't touch the kernel very often) and follow up on lkml and/or here.
Alexey
