On Mon, 2017-10-02 at 10:55 -0400, Theodore Ts'o wrote: > On Mon, Oct 02, 2017 at 02:23:02PM +0000, Joakim Tjernlund wrote: > > Hi ext4 devs > > > > Adding the patch last in this mail cause lots of build errors in ext4, here is a few: > > Why did you need this patch to fix problems in VirtualBox? Earlier VirtualBox had some c++ code that included kernel header and g++ is much pickier about these things so I had to do some cleanups to build some of the VirtualBox modules. I think it is better now but I haven't really checked. > > Cleaning this up is going to be a little tricky, because one of the > implications the void * declaration in the __set_bit_le() declaration > is that there isn't any particular alignment requirement with the __le > functions. But the long * declaration implies that the bitmaps have > to be aligned to sizeof(long). > > For the ext4 bitmap, we use it on bh->b_data, for which we can safely > assume is long-aligned. But the mballoc buddy bitmaps use > mb_set_bit() in ways that are _not_ guaranteed to be long aligned. > > So fixing this is going to be a bit painful, and will likely result in > a performance regression for ext4. We can make our own version that > open codes it as C functions --- but then we lose all of the > architecture optimized bitop functions. > > 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? > > 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. Jocke
