函数xdes_set_bit

/**********************************************************************//**
Sets a descriptor bit of a page. */
UNIV_INLINE
void
xdes_set_bit(
/*=========*/
    xdes_t*    descr,    /*!< in: descriptor */
    ulint    bit,    /*!< in: XDES_FREE_BIT or XDES_CLEAN_BIT */
    ulint    offset,    /*!< in: page offset within extent:
            0 ... FSP_EXTENT_SIZE - 1 */
    ibool    val,    /*!< in: bit value */
    mtr_t*    mtr)    /*!< in: mtr */
{
    ulint    index;
    ulint    byte_index;
    ulint    bit_index;
    ulint    descr_byte;

    ut_ad(mtr_memo_contains_page(mtr, descr, MTR_MEMO_PAGE_X_FIX));
    ut_ad((bit == XDES_FREE_BIT) || (bit == XDES_CLEAN_BIT));
    ut_ad(offset < FSP_EXTENT_SIZE);

    index = bit + XDES_BITS_PER_PAGE * offset;

    byte_index = index / ;
    bit_index = index % ;
    /**     *XDES_BITMAP是个8行8列的表格     *mtr_read_ulint(desc+XDES_BITMAP+byte_index, MLOG_1BYTE,mtr)可取出第N行的数据     *ut_bit_set_nth将第M位置1,并与上面的数据做|运算     */
    descr_byte = mtr_read_ulint(descr + XDES_BITMAP + byte_index,MLOG_1BYTE, mtr);
    descr_byte = ut_bit_set_nth(descr_byte, bit_index, val);//val为false 详见

    mlog_write_ulint(descr + XDES_BITMAP + byte_index, descr_byte,MLOG_1BYTE, mtr);
}