memblock: free_unused_memmap: use pageblock units instead of MAX_ORDER

stable inclusion
from stable-v5.10.87
commit b4b54c7ba149ffa2dcb11b2a84ebf20189ff7f89
bugzilla: 186049 https://gitee.com/openeuler/kernel/issues/I4QVYL

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b4b54c7ba149ffa2dcb11b2a84ebf20189ff7f89

--------------------------------

[ Upstream commit e2a86800 ]

The code that frees unused memory map uses rounds start and end of the
holes that are freed to MAX_ORDER_NR_PAGES to preserve continuity of the
memory map for MAX_ORDER regions.

Lots of core memory management functionality relies on homogeneity of the
memory map within each pageblock which size may differ from MAX_ORDER in
certain configurations.

Although currently, for the architectures that use free_unused_memmap(),
pageblock_order and MAX_ORDER are equivalent, it is cleaner to have common
notation thought mm code.

Replace MAX_ORDER_NR_PAGES with pageblock_nr_pages and update the comments
to make it more clear why the alignment to pageblock boundaries is
required.
Signed-off-by: NMike Rapoport <rppt@linux.ibm.com>
Tested-by: NTony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/lkml/20210630071211.21011-1-rppt@kernel.org/
[backport upstream modification in mm/memblock.c to arch/arm/mm/init.c]
Signed-off-by: NMark-PK Tsai <mark-pk.tsai@mediatek.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

arch/arm/mm/init.c

@@ -315,11 +315,11 @@ static void __init free_unused_memmap(void)
 				 ALIGN(prev_end, PAGES_PER_SECTION));
 #else
 		/*
-		 * Align down here since the VM subsystem insists that the
-		 * memmap entries are valid from the bank start aligned to
-		 * MAX_ORDER_NR_PAGES.
+		 * Align down here since many operations in VM subsystem
+		 * presume that there are no holes in the memory map inside
+		 * a pageblock
 		 */
-		start = round_down(start, MAX_ORDER_NR_PAGES);
+		start = round_down(start, pageblock_nr_pages);
 #endif
 		/*
 		 * If we had a previous bank, and there is a space
@@ -329,11 +329,11 @@ static void __init free_unused_memmap(void)
 			free_memmap(prev_end, start);
 
 		/*
-		 * Align up here since the VM subsystem insists that the
-		 * memmap entries are valid from the bank end aligned to
-		 * MAX_ORDER_NR_PAGES.
+		 * Align up here since many operations in VM subsystem
+		 * presume that there are no holes in the memory map inside
+		 * a pageblock
 		 */
-		prev_end = ALIGN(end, MAX_ORDER_NR_PAGES);
+		prev_end = ALIGN(end, pageblock_nr_pages);
 	}
 
 #ifdef CONFIG_SPARSEMEM