Hi,
I have some time back started reading LINUX VM code.
From /usr/src/linux/mm/page_alloc.c
I find these lines in free_area_init_core()
Page buddy system uses "index >> (i+1)"
In a 16 page system
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0 order 0 1 2 3 4 5 6 7
1 order 0 1 2 3
2 order 0 1
3 order 0
Hence idx = page >> ( order + 1 )
For page 10
+++++++++++
0 order idx = 10 >> ( 0 + 1 ) = 5th bit
1 order idx = 10 >> ( 1 + 1 ) = 2nd bit
2 order idx = 10 >> ( 2 + 1 ) = 1st bit
3 order idx = 10 >> ( 3 + 1 ) = 0th bit
I can understand the computation logic used by buddy system.
However I could not follow remaining part of the comment.
bitmap_size = (size-1) >> (i+4);
i takes value for 0 to 9 since MAX_ORDER is 10 .
MAX_ORDER is 10 because the order can span from 0 to 9
Here size is zone size,for example zone_dma has a value 4096.
Can anyone please help me in understanding how the bitmap size
is calculated??
Thanks,
Sudharsan.
