mm/compact: why use low watermark to determine whether compact is finished instead of use high watermark?

[Xishi Qiu]

Hi,

I find all the watermarks in mm/compaction.c are low_wmark_pages(),
so why not use high watermark to determine whether compact is finished?

e.g. 
__alloc_pages_nodemask()
        get_page_from_freelist()
        this is fast path, use use low_wmark_pages() in __zone_watermark_ok()

        __alloc_pages_slowpath()
        this is slow path, usually use min_wmark_pages()

kswapd
        balance_pgdat()
        use high_wmark_pages() to determine whether zone is balanced

Thanks,
Xishi Qiu