mm: CMA reservations require 32MiB alignment in 16KiB page size kernels instead of 8MiB in 4KiB page size kernel.

[Juan Yescas <jyescas@xxxxxxxxxx>]

Hi Linux memory team

When the drivers reserve CMA memory in 16KiB kernels, the minimum
alignment is 32 MiB as per CMA_MIN_ALIGNMENT_BYTES. However, in 4KiB
kernels, the CMA alignment is 4MiB.

This is forcing the drivers to reserve more memory in 16KiB kernels,
even if they only require 4MiB or 8MiB.

reserved-memory {
      #address-cells = <2>;
      #size-cells = <2>;
      ranges;
      tpu_cma_reserve: tpu_cma_reserve {
            compatible = "shared-dma-pool";
            reusable;
           size = <0x0 0x2000000>; /* 32 MiB */
}

One workaround to continue using 4MiB alignment is:

- Disable CONFIG_TRANSPARENT_HUGEPAGE so the buddy allocator does NOT
have to allocate huge pages (32 MiB in 16KiB page sizes)
- Set ARCH_FORCE_MAX_ORDER for ARM64_16K_PAGES to "8", instead of
"11", so CMA_MIN_ALIGNMENT_BYTES is equals to 4 MiB

    config ARCH_FORCE_MAX_ORDER
        int
        default "13" if ARM64_64K_PAGES
        default "8" if ARM64_16K_PAGES
       default "10"

#define MAX_PAGE_ORDER CONFIG_ARCH_FORCE_MAX_ORDER      // 8
#define pageblock_order MAX_PAGE_ORDER              // 8
#define pageblock_nr_pages (1UL << pageblock_order)    // 256
#define CMA_MIN_ALIGNMENT_PAGES pageblock_nr_pages      // 256
#define CMA_MIN_ALIGNMENT_BYTES (PAGE_SIZE * CMA_MIN_ALIGNMENT_PAGES)
  // 16384 * 256 = 4194304 = 4 MiB

After compiling the kernel with this changes, the kernel boots without
warnings and the memory is reserved:

[    0.000000] Reserved memory: created CMA memory pool at
0x000000007f800000, size 8 MiB
[    0.000000] OF: reserved mem: initialized node tpu_cma_reserve,
compatible id shared-dma-pool
[    0.000000] OF: reserved mem:
0x000000007f800000..0x000000007fffffff (8192 KiB) map reusable
tpu_cma_reserve

#  uname -a
Linux buildroot 6.12.9-dirty
# zcat /proc/config.gz | grep ARM64_16K
CONFIG_ARM64_16K_PAGES=y
# zcat /proc/config.gz | grep TRANSPARENT_HUGE
CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE=y
# CONFIG_TRANSPARENT_HUGEPAGE is not set
# cat /proc/pagetypeinfo
Page block order: 8
Pages per block:  256

Free pages count per migrate type at order       0      1      2
3      4      5      6      7      8
Node    0, zone      DMA, type    Unmovable      1      1     13
6      5      2      0      0      1
Node    0, zone      DMA, type      Movable      9     16     19
13     13      5      2      0    182
Node    0, zone      DMA, type  Reclaimable      0      1      0
1      1      0      0      1      0
Node    0, zone      DMA, type   HighAtomic      0      0      0
0      0      0      0      0      0
Node    0, zone      DMA, type          CMA      1      0      0
0      0      0      0      0     49
Node    0, zone      DMA, type      Isolate      0      0      0
0      0      0      0      0      0
Number of blocks type     Unmovable      Movable  Reclaimable
HighAtomic          CMA      Isolate
Node 0, zone      DMA            6          199            1
 0           50            0


However, with this workaround, we can't use transparent huge pages.

Is the CMA_MIN_ALIGNMENT_BYTES requirement alignment only to support huge pages?
Is there another option to reduce the CMA_MIN_ALIGNMENT_BYTES alignment?

Thanks
Juan