Is there any way to limit the amount of data in an mbuf chain submitted to a driver?

[Richard Sharpe]

Hi folks,

I understand better why I am seeing EINVAL intermittently when sending
data from Samba via SMB2.

The ixgbe driver, for TSO reasons, limits the amount of data that can
be DMA'd to 65535 bytes. It returns EINVAL for any mbuf chain larger
than that.

The SO_SNDBUF for that socket is set to 131972. Mostly there is less
than 64kiB of space available, so that is all TCP etc can put into the
socket in one chain of mbufs. However, every now and then there is
more than 65535 bytes available in the socket buffers, and we have an
SMB packet that is larger than 65535 bytes, and we get hit.

To confirm this I am going to set SO_SNDBUF back to the default of
65536 and test again. My repros are very reliable.

However, I wondered if my only way around this if I want to continue
to use SO_SNDBUF sizes larger than 65536 is to fragment large mbuf
chains in the driver?

-- 
Regards,
Richard Sharpe
(何以解憂？唯有杜康。--曹操)