ANDROID: mm: memblock: avoid to create memmap for memblock nomap regions

This 'commit 86588296acbf ("fdt: Properly handle "no-map" field in the
memory region")' is keeping the no-map regions in memblock.memory with
MEMBLOCK_NOMAP flag set to use no-map memory for EFI using memblock
api's, but during the initialization sparse_init mark all memblock.memory
as present using for_each_mem_pfn_range, which is creating the memmap for
no-map memblock regions.

Upstream has suggested to make use of bootloader to pass this as not a
memory,but because of possibility that some bootloaders might not support
this and also due to time constraints in evaluating this approach on 5.10,
Use command line parameter as a temporary solution. Get in the appropriate
solution later after further discussion with upstream.

Add kernel param "android12_only.will_be_removed_soon.memblock_nomap_remove"
which when enabled will remove page structs for these regions using memblock_remove.
With this change we will be able to save ~11MB memory for ~612MB carve out.

android12_only.will_be_removed_soon.memblock_nomap_remove=true:
[    0.000000] memblock_alloc_exact_nid_raw: 115343360 bytes
align=0x200000 nid=0 from=0x0000000080000000 max_addr=0x0000000000000000
sparse_buffer_init+0x60/0x8c
[    0.000000] memblock_reserve: [0x0000000932c00000-0x00000009399fffff]
memblock_alloc_range_nid+0xbc/0x1a0
[    0.000000] On node 0 totalpages: 1627824
[    0.000000] DMA32 zone: 5383 pages used for memmap
[    0.000000] Normal zone: 20052 pages used for memmap

Default or android12_only.will_be_removed_soon.memblock_nomap_remove=false:
[    0.000000] memblock_alloc_exact_nid_raw: 117440512 bytes
align=0x200000 nid=0 from=0x0000000080000000 max_addr=0x0000000000000000
sparse_buffer_init+0x60/0x8c
[    0.000000] memblock_reserve: [0x0000000932a00000-0x00000009399fffff]
memblock_alloc_range_nid+0xbc/0x1a0
[    0.000000] On node 0 totalpages: 1788416
[    0.000000] DMA32 zone: 8192 pages used for memmap
[    0.000000] Normal zone: 20052 pages used for memmap.

Change-Id: I34a7d46f02a6df7c769af3e53e44e49d6fc515af
Bug: 227974747
Link: https://lore.kernel.org/all/20210115172949.GA1495225@robh.at.kernel.org
Signed-off-by: Faiyaz Mohammed <quic_faiyazm@quicinc.com>
Signed-off-by: Vijayanand Jitta <quic_vjitta@quicinc.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -2533,6 +2533,12 @@
 
 	memblock=debug	[KNL] Enable memblock debug messages.
 
+	android12_only.will_be_removed_soon.memblock_nomap_remove= [KNL]
+			Setting this to true through kernel command line will
+			call memblock_remove on the regions marked with no-map
+			property thereby saving memory by removing page structs
+			for those regions. By default this is set to false.
+
 	load_ramdisk=	[RAM] [Deprecated]
 
 	lockd.nlm_grace_period=P  [NFS] Assign grace period.