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On+FoMeg/4qxjuubcLiqZ1GG96ZrVdKf9+QHxV74r9hyY21sG0wACIvGpz7npW5Tqe4eUhF/y87CFmBR5oeyXw6HslQSjI3Foh6QbnX9SrpNjiK8xNOPHahMSRKoqB+mEoy0GjeUfXH2IHmRrd1tAZc3E3/PEZPuIDbJfbjOC0Q1nyGz0r6DiApJdt2s44rY+2G4Q3e7hNPIZtFyHPe2kzJpaqVXYA5AW6SfTceGSkpH9d2vuSM9uhHdefK1RBbB87C2PmknzFIiC8gEGSEj3xjfhM3jcJxTbOrJQOHfUBwZQ8+D8qnGIZcAQ3M3E+UQyIqL38Bn2W/dk9yoNQFCJOnEIGvzYZeL2uhEiPpUoAv/YxVyGZcAt8Snr+w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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