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? 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. 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. Cheers, - Ted
