+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?
> Is there another option to reduce the CMA_MIN_ALIGNMENT_BYTES alignment?
>
> Thanks
> Juan
