Re: [PATCH] zerocopy NFS updated

[Andi Kleen]

Hirokazu Takahashi <[e-mail]> writes:

> But I wonder about sendpage. I guess HW IP checksum for outgoing
> pages might be miscalculated as VFS can update them anytime.
> New feature like COW pagecache should be added to VM and they 
> should be duplicated in this case.

For hw checksums it should not be a problem. NICs usually load
the packet into their packet fifo and compute the checksum on the fly
and then patch it into the header in the fifo before sending it out. A
NIC that would do slow PCI bus mastering twice just to compute the checksum
would be very dumb and I doubt they exist (if yes I bet it would be
faster to do software checksumming on them). When the NIC only
accesses the memory once there is no race window.

-Andi