[PATCH v2 1/3] arm64: perf: Skip device memory during user callchain unwinding

[Fredrik Markstrom]

As pointed out by Sashiko there is a TOCTOU race between the page
table walk (which checks for device memory) and the
copy_from_user_inatomic (which reads the frame). Another thread
could mmap device memory at the target address in between.

Three options:

1. Accept the race — it requires a cooperating thread with precise
   timing, and the guard still eliminates the common case (stable
   device mappings).
2. Use mmap_read_trylock() once per unwind, held across all frames:
   a. Fall back to the lockless walk (best-effort) when the trylock fails.
   b. Stop unwinding user space entirely when the trylock fails.

Option 2 adds mmap_lock contention per unwind. Is the race narrow
enough to accept, or is the trylock approach preferred?