On Windows, real thread stacks that exist before the guest arena is
reserved can sit near the arena top, so the AMM range correctly lands
9-60 GiB below it and the 1 GiB bound failed about 1 run in 10. The test
now requires that no free 64 GiB window exists above the AMM range.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
The AMM ranges were reserved at the last 64 GiB of the guest arena and
the reservation threw when that window was not free. On Windows the
stacks of the threads that exist before the arena is reserved (the main
thread and the loader's workers) are placed at random addresses and are
left in the arena as host regions; when one fell in the last 64 GiB, the
first AMM use threw "fixed guest mapping ... overlaps a host region in
the guest arena". Reserve the highest free 64 GiB window instead: the
last 64 GiB when it is free, and the window just below an occupied part
of it otherwise.
screamm
Zurisen