about arm_add_memory

[wangjun]

in  /arch/arm/kernel/setup.c


static int __init arm_add_memory(unsigned long start, unsigned long size)
...
size -= start & ~PAGE_MASK;
bank->start = PAGE_ALIGN(start);
bank->size  = size & PAGE_MASK;
bank->node  = PHYS_TO_NID(start);
.....

what   size -= start & ~PAGE_MASK  to do ?
thanks!













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