PAGE_ALIGN()

将物理地址addr修整为页边界地址(页的上边界)

#define PAGE_ALIGN(addr)    (((addr)+PAGE_SIZE-1)&PAGE_MASK)

|------------|<-- PAGE_ALGN(addr)
|            |
|            |
|            |
|            |<-- addr
|            |
|            |
|            |
|            |
|------------|
one page(4K)




#define PAGE_SHIFT 12
#define PAGE_SIZE   (1UL << PAGE_SHIFT)
#define PAGE_MASK   (~(PAGE_SIZE-1))



PAGE_MASK = ~(1 0000 0000 0000 - 1) = ~(1111 1111 1111) = 0000 0000 0000 
PAGE_SIZE = 1 0000 0000 0000        = 2^12              = 4K


如addr为0x22000001 。。PAGE_ALIGN(addr)=(0x22000001+4096-1)&0xfffff000
=(0x22000001+0xfff)&0xfffff000=0x22001000&0xfffff000
=0x22001000;
同样，比如addr为0x22000003，PAGE_ALIGN(addr)后仍然是0x22001000；

就是和下一个页对齐，一个页为4k。起始地址为0x xxxxx 000；