函数功能:获取barrier bucket对齐之后的扇区长度。
源码调用:
good_sectors = align_to_barrier_unit_end(sector_nr, good_sectors);
其中good_sectors的值为128,也就是缺省同步chunk size 64K,kretprobe跟踪的结果来看,跟踪到的值为128?这个值是固定的? - - 不是。
函数原型:static sector_t align_to_barrier_unit_end(sector_t start_sector, sector_t sectors)
参数:
返回值:可操作扇区的范围。
static sector_t align_to_barrier_unit_end(sector_t start_sector,
sector_t sectors)
{
sector_t len;
WARN_ON(sectors == 0);
/*
* len is the number of sectors from start_sector to end of the
* barrier unit which start_sector belongs to.
*/
/* len 是start_sector所属的屏障单元从start_sector到末端的扇区数。 */
len = round_up(start_sector + 1, BARRIER_UNIT_SECTOR_SIZE) -
start_sector;
if (len > sectors)
len = sectors;
return len;
}
/*
* This looks more complex than it should be. But we need to
* get the type for the ~ right in round_down (it needs to be
* as wide as the result!), and we want to evaluate the macro
* arguments just once each.
*/
#define __round_mask(x, y) ((__typeof__(x))((y)-1))
#define round_up(x, y) ((((x)-1) | __round_mask(x, y))+1)
#define round_down(x, y) ((x) & ~__round_mask(x, y))
举个例子,假设要对齐的起始扇区号为7,BARRIER_UNIT_SECTOR_SIZE的值为1<<17。
A = __round_mask = (1<<17) - 1 = 0 1111 1111 1111 1111
B = x - 1 = 8 - 1 = 0111
(A | B = 0 1111 1111 1111 1111) + 1 = 1 0000 0000 0000 0000
所以此时,round_up的返回值依然为BARRIER_UNIT_SECTOR_SIZE。
那么如果当起始扇区号大于BARRIER_UNIT_SECTOR_SIZE会是怎么样呢?假设起始扇区号为x = 1 0000 0000 0000 0010
。
A = __round_mask = (1<<17) - 1 = 0 1111 1111 1111 1111
B = x - 1 = 1 0000 0000 0000 0001
(A | B = 1 1111 1111 1111 1111) + 1 = 10 0000 0000 0000 0000
所以此时,round_up的返回值为BARRIER_UNIT_SECTOR_SIZE << 1,就是1<<18,是BARRIER_UNIT_SECTOR_SIZE 的两倍。
所以round_up
宏的作用是返回给定数 x
向上舍入到最接近的 y
的倍数。
整明白round_up函数的能力之后,再看看align_to_barrier_unit_end这个函数。
len = round_up(start_sector + 1, BARRIER_UNIT_SECTOR_SIZE) - start_sector;
len的值是取到最接近BARRIER_UNIT_SECTOR_SIZE整数倍的值,然后再减去扇区的起始扇区的值,len的值就是是start_sector所属的屏障单元从start_sector到bucket末端的扇区数。
+---------+--------barrier bucket------------+------------+
| | | |
| start sector bucket_end - sectors |
+---------+--------barrier bucket------------+------------+
if (len > sectors)
len = sectors;