Normal should work just as well.
> Huh? Like this?
> 1: mmap(), write 4MB
> 2: open(), ftruncate() - and close() or not?
As two processes - the first mmaps a data area for read then writes it to
another file. This takes page faults during the write (paging in the memory
we mmap'd). The second process does an ftruncate which revokes the memory
from under the first process mmap during a page fault: What you get therefore
is ext2fs suddenely trying to write from non existant pages and "Oops..."
messages galore as well as a mess that e2fsck moans about a fair bit
but seems to fix.
Alan