Man page 9 : __bitmap_shift_right

__bitmap_shift_right - logical right shift of the bits in a bitmap

void __bitmap_shift_right(unsigned long *  dst , const unsigned long *  src , int  shift , int  bits );

dst
    destination bitmap

src
    source bitmap

shift
    shift by this many bits

bits
    bitmap size, in bits

Shifting right (dividing) means moving bits in the MS -> LS bit direction. Zeros are fed into the vacated MS positions and the LS bits shifted off the bottom are lost.