arm64: make ARCH_FORCE_MAX_ORDER selectable

The other architectures with ARCH_FORCE_MAX_ORDER are selectable,
but not for ARM64, this is to make it selectable on ARM64, which
is useful for user that need to allocate more than 4MB of physically
contiguous memory with 4K pagesize, also bigger on 16K pagesize too, the
max value of MAX_ORDER is calculated bellow,

see include/linux/mmzone.h,

  MAX_ORDER - 1 + PAGE_SHIFT <= SECTION_SIZE_BITS

  so max value of MAX_ORDER = SECTION_SIZE_BITS + 1 - PAGE_SHIFT

    | SECTION_SIZE_BITS |  PAGE_SHIFT  |  max MAX_ORDER  |  default MAX_ORDER |
----+-------------------+--------------+-----------------+--------------------+
4K  |       27          |      12      |     16          |      11            |
16K |       27          |      14      |     14          |      12            |
64K |       29          |      16      |     14          |      14            |
----+-------------------+--------------+-----------------+--------------------+

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Link: https://lore.kernel.org/r/20230104130000.69806-1-wangkefeng.wang@huawei.com
[catalin.marinas@arm.com: add the calculations as comment to arch/arm64/Kconfig]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 0393480..8cfd8dc 100644 (file)
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1456,10 +1456,23 @@ config XEN
        help
          Say Y if you want to run Linux in a Virtual Machine on Xen on ARM64.
 
+# include/linux/mmzone.h requires the following to be true:
+#
+#   MAX_ORDER - 1 + PAGE_SHIFT <= SECTION_SIZE_BITS
+#
+# so the maximum value of MAX_ORDER is SECTION_SIZE_BITS + 1 - PAGE_SHIFT:
+#
+#     | SECTION_SIZE_BITS |  PAGE_SHIFT  |  max MAX_ORDER  |  default MAX_ORDER |
+# ----+-------------------+--------------+-----------------+--------------------+
+# 4K  |       27          |      12      |       16        |         11         |
+# 16K |       27          |      14      |       14        |         12         |
+# 64K |       29          |      16      |       14        |         14         |
 config ARCH_FORCE_MAX_ORDER
-       int
+       int "Maximum zone order" if ARM64_4K_PAGES || ARM64_16K_PAGES
        default "14" if ARM64_64K_PAGES
+       range 12 14 if ARM64_16K_PAGES
        default "12" if ARM64_16K_PAGES
+       range 11 16 if ARM64_4K_PAGES
        default "11"
        help
          The kernel memory allocator divides physically contiguous memory