Again, if you did it this way, the lower MPOL_FLAG_SHIFT bits of the new
'flags' member would always be zero if you are still going to use the
MPOL_F_* defines from linux/mempolicy.h to do your bit testing.
I do not subscribe to the theory that just because we have a couple extra
bytes of space somewhere in struct mempolicy that we have to use it
immediately.
It makes the kernel code simpler, in a way.
Now we only have to pass a single actual among functions that include both
the mode and optional flags (there are a lot of them and they span not
only the VM but also filesystem code). The catch is that we have to use a
mpol_mode() wrapper for mode conditionals or switch statements.
But testing the flags is just as easy as
if (mode & MPOL_F_STATIC_NODES) {
...
}
That test would remain unchanged (except for s/mode/flags/) if flags were
stored in a separate member.
So by storing them both in an 'unsigned short' member of struct mempolicy:
- we don't use any additional memory (and we can use those two extra
bytes you identified earlier later), and
- we only have to pass a single actual to many different functions that
require both the mode and optional mode flags.
--
