Only present if CONFIG_HIGHMEM is set.
This defaults to 0 (false), meaning that the dirty_ratio and dirty_background_ratio ratios are calculated as a percentage of low memory only. This protects against excessive scanning in page reclaim, swapping and general VM distress.
Setting this to 1 can be useful on 32 bit machines where you want to make random changes within an memory mapped file that is larger than your available low memory without causing large quantities of random IO. It is safe if the behavior of all programs running on the machine is known and memory will not be otherwise stressed.