file: /proc/sys/vm/highmem_is_dirtyable
variable: vm.highmem_is_dirtyable
Official reference

Available only for systems with CONFIG_HIGHMEM enabled (32b systems).

This parameter controls whether the high memory is considered for dirty writers throttling. This is not the case by default which means that only the amount of memory directly visible/usable by the kernel can be dirtied. As a result, on systems with a large amount of memory and lowmem basically depleted writers might be throttled too early and streaming writes can get very slow.

Changing the value to non zero would allow more memory to be dirtied and thus allow writers to write more data which can be flushed to the storage more effectively. Note this also comes with a risk of pre-mature OOM killer because some writers (e.g. direct block device writes) can only use the low memory and they can fill it up with dirty data without any throttling.