On 19 Jan 2025, at 19:17, Barry Song wrote:
> On Sat, Jan 18, 2025 at 12:00 PM Juan Yescas <jyescas@xxxxxxxxxx> 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.
>
> I don’t think this is accurate. You can still use mTHP with a size
> equal to or smaller than 4MiB,
> right?
>
> By the way, what specific regression have you observed when reserving
> a larger size like
> 32MB?
> For CMA, the over-reserved memory is still available to the system for
> movable folios. 28MiB
The fallbacks table does not have MIGRATE_CMA as a fallback for any
migratetype. How can it be used for movable folios? Am I missing something?
> doesn’t seem significant enough to cause a noticeable regression, does it?
--
Best Regards,
Yan, Zi
