RE: [PATCH] rds: Error on offset mismatch if not loopback

[Venkat Venkatsubra <venkat.x.venkatsubra@oracle.com>]

We now have lot more information than we did before.
When sending a "congestion update" in rds_ib_xmit() we are now returning an incorrect number as bytes sent:

        BUG_ON(off % RDS_FRAG_SIZE);
        BUG_ON(hdr_off != 0 && hdr_off != sizeof(struct rds_header));

        /* Do not send cong updates to IB loopback */
        if (conn->c_loopback
            && rm->m_inc.i_hdr.h_flags & RDS_FLAG_CONG_BITMAP) {
                rds_cong_map_updated(conn->c_fcong, ~(u64) 0);
                scat = &rm->data.op_sg[sg];
                ret = sizeof(struct rds_header) + RDS_CONG_MAP_BYTES;
                ret = min_t(int, ret, scat->length - conn->c_xmit_data_off);
                return ret;
        }

It returns min(8240, 4096-0) i.e. 4096 bytes.
The caller rds_send_xmit() is made to think a partial message (4096 out of 8240) was sent.
It calls rds_ib_xmit() again with a data offset "off" of 4096-48 (rds header) (=4048 bytes). And we hit the BUG_ON.

The reason I didn't hit the panic on my test on Oracle UEK2 which is based on 2.6.39 kernel is it had it like this:
        BUG_ON(off % RDS_FRAG_SIZE);
        BUG_ON(hdr_off != 0 && hdr_off != sizeof(struct rds_header));

        /* Do not send cong updates to IB loopback */
        if (conn->c_loopback
            && rm->m_inc.i_hdr.h_flags & RDS_FLAG_CONG_BITMAP) {
                rds_cong_map_updated(conn->c_fcong, ~(u64) 0);
                return sizeof(struct rds_header) + RDS_CONG_MAP_BYTES;
        }
(So it wasn't 100% 2.6.39 ;-). )
It returned 8240 bytes. The caller rds_send_xmit decides the full message was sent (48 byte header + 4096 data + 4096 data).
And it worked.

Then I found this info on the change that was done upstream which now causes the panic:
http://marc.info/?l=linux-netdev&m=129908332903057
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=6094628bfd94323fc1cea05ec2c6affd98c18f7f 

Will investigate more into which problem the above change addressed.

Venkat