Re: [PATCH] mm: Add ARCH_FORCE_PAGE_BLOCK_ORDER to select page block order

On 1 May 2025, at 14:21, Kalesh Singh wrote:

On Thu, May 1, 2025 at 10:11 AM Juan Yescas [off-list ref] wrote:

quoted

On Thu, May 1, 2025 at 7:24 AM Zi Yan [off-list ref] wrote:

quoted

On 1 May 2025, at 1:25, Juan Yescas wrote:

quoted

Problem: On large page size configurations (16KiB, 64KiB), the CMA
alignment requirement (CMA_MIN_ALIGNMENT_BYTES) increases considerably,
and this causes the CMA reservations to be larger than necessary.
This means that system will have less available MIGRATE_UNMOVABLE and
MIGRATE_RECLAIMABLE page blocks since MIGRATE_CMA can't fallback to them.

The CMA_MIN_ALIGNMENT_BYTES increases because it depends on
MAX_PAGE_ORDER which depends on ARCH_FORCE_MAX_ORDER. The value of
ARCH_FORCE_MAX_ORDER increases on 16k and 64k kernels.

For example, the CMA alignment requirement when:

- CONFIG_ARCH_FORCE_MAX_ORDER default value is used
- CONFIG_TRANSPARENT_HUGEPAGE is set:

PAGE_SIZE | MAX_PAGE_ORDER | pageblock_order | CMA_MIN_ALIGNMENT_BYTES
-----------------------------------------------------------------------
   4KiB   |      10        |      10         |  4KiB * (2 ^ 10)  =  4MiB
  16Kib   |      11        |      11         | 16KiB * (2 ^ 11) =  32MiB
  64KiB   |      13        |      13         | 64KiB * (2 ^ 13) = 512MiB

There are some extreme cases for the CMA alignment requirement when:

- CONFIG_ARCH_FORCE_MAX_ORDER maximum value is set
- CONFIG_TRANSPARENT_HUGEPAGE is NOT set:
- CONFIG_HUGETLB_PAGE is NOT set

PAGE_SIZE | MAX_PAGE_ORDER | pageblock_order |  CMA_MIN_ALIGNMENT_BYTES
------------------------------------------------------------------------
   4KiB   |      15        |      15         |  4KiB * (2 ^ 15) = 128MiB
  16Kib   |      13        |      13         | 16KiB * (2 ^ 13) = 128MiB
  64KiB   |      13        |      13         | 64KiB * (2 ^ 13) = 512MiB

This affects the CMA reservations for the drivers. If a driver in a
4KiB kernel needs 4MiB of CMA memory, in a 16KiB kernel, the minimal
reservation has to be 32MiB due to the alignment requirements:

reserved-memory {
    ...
    cma_test_reserve: cma_test_reserve {
        compatible = "shared-dma-pool";
        size = <0x0 0x400000>; /* 4 MiB */
        ...
    };
};

reserved-memory {
    ...
    cma_test_reserve: cma_test_reserve {
        compatible = "shared-dma-pool";
        size = <0x0 0x2000000>; /* 32 MiB */
        ...
    };
};

Solution: Add a new config ARCH_FORCE_PAGE_BLOCK_ORDER that
allows to set the page block order. The maximum page block
order will be given by ARCH_FORCE_MAX_ORDER.

Why not use a boot time parameter to change page block order?

That is a good option. The main tradeoff is:

- The bootloader would have to be updated on the devices to pass the right
pageblock_order value depending on the kernel page size. Currently,
We can boot 4k/16k kernels without any change in the bootloader.

Once we change the page block order we likely need to update the CMA
reservations in the device tree to match the new min alignment, which
needs to be recompiled and flashed to the device. So there is likely
not a significant process saving by making the page block order a boot
parameter.

Got it. Thank you for the explanation.

--
Best Regards,
Yan, Zi