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

[Zi Yan]

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

> 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?
Otherwise, you will need to maintain an additional kernel
binary for your use case.

--
Best Regards,
Yan, Zi