[2.5] [PATCH] Don't loop looking for free SLB entries in do_slb_bolted

[Olof Johansson]

Anton and I have done some work in 2.4 to optimize the SLB reload path.
One big time waster for a busy system is the search for a free entry.
This is the corresponding patch for 2.5/2.6. It's smaller since we don't
have to do slbie's on 2.6.

For a system with smaller working set than the SLB can fit (<16GB),
evicting a used entry is no biggie: It'll be fauled in again and we'll
reach a stable state after a few iterations through this. For systems
with larger working sets, there is no stable state and we will save alot
of time by not scanning all 63 entries on every fault just to find them
all valid and fall back to round-robin.

Comments are welcome. I haven't tested the 2.6 patch as much as 2.4 due
to lack of big hardware and good workloads.

-Olof
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