On Tue, 2017-02-28 at 14:52 -0800, Andy Lutomirski wrote: > The user pages are a gift to the kernel. The application may not > modify this memory ever, otherwise the page cache and on-disk data may > differ. > > This is just not okay IMO.
TCP works just fine in this case. TX checksum will be computed by the NIC after/while data is copied. If really the application changes the data, that will not cause any problems, other than user side consistency. This is why we require a copy (for all buffers that came from zero-copy) if network stack hits a device that can not offload TX checksum. Even pwrite() does not guarantee consistency if multiple threads are using it on overlapping regions.
