memmap linux kernel parameters are:
mem=nn[KMG] [KNL,BOOT] Force usage of a specific amount of memory
Amount of memory to be used when the kernel is not able
to see the whole system memory or for test.
[IA-32] Use together with memmap= to avoid physical
address space collisions. Without memmap= PCI devices
could be placed at addresses belonging to unused RAM.
mem=nopentium [BUGS=IA-32] Disable usage of 4MB pages for kernel
memory.
memmap=exactmap [KNL,IA-32] Enable setting of an exact
E820 memory map, as specified by the user.
Such memmap=exactmap lines can be constructed based on
BIOS output or other requirements. See the memmap=nn@ss
option description.
memmap=nn[KMG]@ss[KMG]
[KNL] Force usage of a specific region of memory
Region of memory to be used, from ss to ss+nn.
memmap=nn[KMG]#ss[KMG]
[KNL,ACPI] Mark specific memory as ACPI data.
Region of memory to be used, from ss to ss+nn.
memmap=nn[KMG]$ss[KMG]
[KNL,ACPI] Mark specific memory as reserved.
Region of memory to be used, from ss to ss+nn.
When passing via grub you need to escape the $:
ie: memmap 128M\$1024M
Without the escaping, memmap doesn't behave correctly.
In /etc/default/grub you need to escape \ too.
So option as to be writed this way: memmap 128M\\\$512M
Several memmap options could be used as the same time.