OK, I see the problem. SGI's code has sd_raw_{read,write} allocate the
kiobuf then call sd_raw_rw() which issues map_user_kiobuf(). Their
patch also passes an astype variable into sd_raw_rw() which copies the
astype into the kiobuf and right at the bottom layer, map_user_kiobuf()
does different mappings depending on the value of astype. Just a few
layer violations in there.
What I need is to move map_user_kiobuf() up to sd_raw_{read,write} and
make the map call conditional on some external data (filp flag?). Use
map_user_kiobuf() by default or use map_kernel_kiobuf() if the filp is
marked "use kernel pages". That way the syscall code makes the
decision about whether to use kernel or user mappings and passes the
resulting mapped kiobuf to the low level raw I/O code. No decisions in
the low level code about what mappings to use.
So all we need is a map_kernel_kiobuf() routine and a wrapper
map_kiobuf() which takes a flag and calls the relevant map_xxx_kiobuf()
routine. That has a restriction that all the pages in a single kiobuf
are mapped the same way but that is probably acceptable, the kiovec
lets you plug kiobufs together.
Does this sound sensible?
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