On 17 Jan 2025, at 18:00, Juan Yescas wrote:
> + iamjoonsoo.kim@xxxxxxx
> + quic_charante@xxxxxxxxxxx
>
> On Fri, Jan 17, 2025 at 2:52 PM Juan Yescas <jyescas@xxxxxxxxxx> wrote:
>>
>> +Suren Baghdasaryan
>> +Kalesh Singh
>> +T.J. Mercier
>> +Isaac Manjarres
>>
>> On Fri, Jan 17, 2025 at 2:51 PM Juan Yescas <jyescas@xxxxxxxxxx> wrote:
>>>
>>> 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?
No. CMA_MIN_ALIGNMENT_BYTES is limited by CMA_MIN_ALIGNMENT_PAGES, which
is equal to pageblock size. Enabling THP just bumps the pageblock size.
>>> Is there another option to reduce the CMA_MIN_ALIGNMENT_BYTES alignment?
Not easily. CMA is reserved at pageblock level, there is a MIGRATE_CMA
type for a pageblock used for CMA. You will need to change buddy allocator
to allow MIGRATE_CMA pageblocks used by normal page allocations to be able
to reclaim over-reserved CMA memory (and CMA_MIN_ALIGNMENT_BYTES will not
be changed in this case).
You can see more background on this from
patchset: Use pageblock_order for cma and alloc_contig_range alignment,
starting from commit b48d8a8e5ce5 ("mm: page_isolation: move has_unmovable_pages()
to mm/page_isolation.c”)[1].
[1] https://lore.kernel.org/all/20220425143118.2850746-1-zi.yan@xxxxxxxx/
Best Regards,
Yan, Zi
