is remap_pfn_range should align to 2(n) * (page size) ?

[zhuzhenhua <zzh.hust@xxxxxxxxx>]

hello all
           i have a sensor driver want to malloc 2.xM SDRAM to capture data(using DMA),  so i used  remap_pfn_range to malloc 3M.
But in /proc/meminfo, it showes free memory reduce 4M. i also check the /proc/buddyinfo, it seemes too.
(i am looking inside kernel code, but not get clear at now).

 is remap_pfn_range should align to  2(n) * (page size) ?  

thanks for any hints

zzh