An import that should stay watched runs the unwatch hook only when its range
leaves the write watch, after the import. When the range is partly outside the watch
or the walk before the import fails, the range leaves the watch whatever the import
does, so the hook can run first, as it does for the import that does not stay
watched.
After the import, the walk that tells an import's own page marks from CPU writes
(the window compare) is skipped in this case, so the hook collects those marks as CPU
writes. A driver that marks the pages of an import written (the NVIDIA driver on
Windows does, see Pages under an import window) makes the hook keep the blocks they
cover for the CPU unless their bytes are proven unchanged, and those blocks do not
receive the GPU results. A failure of the walk after the import still runs the hook
after it.
The check of #6308 also requires the hook to have run before the import.
AlxcBTW
c20686e114