What the thing really wants is some kind of indication of state,
without having to bloat up the scatterlist structure.
I believe that we have enough of a limited set of accessors to
sg->page that we can more aggressively encode things in the lower
bits.
I'm thinking of encoding the low two bits of sg->page as
follows:
1) bits == 0
then the SG list is linear and sg_next() is sg++
2) bits == 1
the nest SG is an indirect chunk, sg_next() is
therefore something like:
next = *((struct scatterlist **)(sg + 1));
3) bits == 2
this is the last entry in the scatterlist, sg_next() is NULL
So for the cases of ARCH_HAS_SG_CHAIN not being set (ie. back
compatible), we can do no bit encoding in page->flags and just do
sg_next() == sg++, as is done now.
When doing SG chaining, in each non-linear chunk we have to allocate
one more pointer past the end of the scatterlist array (instead of a
full extra scatterlist entry for the indirect pointer encode). Next,
all sg->page accesses have to be guarded to clear the state bits
out first.
I don't know, maybe it would work, and would make the loop termination
issues easier to handle properly.
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