Man page 9 : __bitmap_shift_left

__bitmap_shift_left - logical left shift of the bits in a bitmap

void __bitmap_shift_left(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 left (multiplying) means moving bits in the LS -> MS direction. Zeros are fed into the vacated LS bit positions and those MS bits shifted off the top are lost.