I've done a fairly significant overhaul for cifs to add support for larger rsize= options and the ability for it to do readpages asynchronously. This roughly quadruples read throughput on in basic testing.
The latest upstream patchsets are here:
...but I may need to do another to fix up some merge conflicts with other recently merged code. The first set is an overhaul of the receive codepath to allow for larger receives, and the second changes readpages to use the new code to do readpages.
I suspect that this would be a good thing to include in RHEL6, but we'll have to gauge the benefit over the invasiveness of the patchset.
Handing to Sachin as he has more time to backport this than I do...
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