Actually, I think I was half wrong on that when you asked about
scsi_debug. The scatterlist that sg.c uses is never seen by the block
layer or scsi layer. It is just used as a container to hold segments.
sg.c and st.c use their scatterlist to manage their preallocated
pages/segments. When they pass it to scsi_execute_async, that function
will create a request struct and add bios for the pages.
In 2.6.24 and below, sg.c will send a scatterlist length that does not
match the IO length, and scsi_execute_async will goof up and send a
rq->data_len that does not match the sum of the bios. That is what I was
trying to fix in 2.6.24, but the patch got messed up. In 2.6.25-rc2 and
above that is fixed and scsi_execute_async will catch sg.c doing this
and set rq->data_len and the bio lengths correctly.
So hopefully that helps any fixes you might have planned.
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