On Mon, Oct 02, 2017 at 03:15:33PM +0000, Joakim Tjernlund wrote:

quoted

I believe the reason why the standard bitop functions are made long *
aligned is that on some BE architectures --- I suspect it was PowerPC
but I'm not 100% sure about that --- the native bitop functions
required a long * alignment.  Fortunately all of the little endian
architectures didn't have these alignment restrictions, so we could
keep the __set_bit_le functions to not have any long alignment
restrictions.

If this is a special case for ext4, can you not just do an explicit
type cast in ext4 code?

Sure, it would be safe *today*, but then in the future someone might
change an implementation of the bitop_le* functions for some
architecture which would not tolerate unaligned pointers (since using
a long * would imply this is allowed), and then things would break.

quoted

The fact that bitop and the bitop_le functions are not the same
is... inelegant, but if it represents a practical optimization that is
possible on LE systems but not on BE systems (where bitop_le gets open
coded in C, in an inefficient way, but oh, well, BE systems aren't for
the cool kids anyway :-), I have to ask whether it's really worth it
to do the cleanup.

I see, but by using void * you also loose type checking w.r.t size so
if you by mistake use an u32, you will not notice.

Um, we're never using a u32.  We're using a pointer into a bit array
which is often far larger than 32 or 64 bits.  For example, when we
use a 4k block size, then bh->b_data is a bit array which is 4096*8 ==
32,768 bits.

This is why void * is the right thing --- it's not a u32 or a long.
It's a bit array.  And in the case of the mb buddy bitmap, it's not
necessarily going to start on a a byte boundary which is a multiple of
4 or 8.

					- Ted