tux3: Support mmap write: Fix race of mmap write with truncate(2)

mmap write and truncate(2) can race on delta boundary.

               cpu0                                   cpu1
       delta-1 = get_delta();
                                              mmap write
                                                  delta-2 = get_delta();
                                                  page-B = pagefork(page-A);

If mmap write and truncate(2) ran by above order, truncate(2) see
dirty page for delta-2. We should guarantee one doesn't see data in
future delta.

To avoid this race, this introduces ->truncate_lock. And protects mmap
write while truncate(2) is running.

FIXME: This race would be rare cases in real usage though, more fine
granulate lock would be better.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
4 files changed