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