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SubjectRe: Correlation CMA size and FORCE_MAX_ZONEORDER
On 15.09.22 23:36, Michael Nazzareno Trimarchi wrote:
> Hi all

Hi,

>
> Working on a small device with 128MB of memory and using imx_v6_v7
> defconfig I found that CMA_SIZE_MBYTES, CMA_SIZE_PERCENTAGE
> are not respected. The calculation done does not allow the requested
> size. I think that this should be somehow documented and described but
> I did not
> find the documentation. Does it work this way?
>
> With CMA_SIZE of 8MB I need to have FORCE_MAX_ZONEORDER=12 if I have
> the default FORCE_MAX_ZONEORDER=14 the min size is 32Mb

The underlying constraint is that CMA regions require a certain minimum
alignment+size. They cannot be arbitrarily in size.

CMA_MIN_ALIGNMENT_BYTES expresses that, and corresponds in upstream
kernels to the size of a single pageblock.

In previous kernels, it used to be the size of the largest buddy
allocation granularity (derived from MAX_ORDER, derived from
FORCE_MAX_ZONEORDER).

On upstream kernels, the FORCE_MAX_ZONEORDER constraint should no longer
apply. On most archs, the minimum alignment+size should be 2 MiB
(x86-64, aarch64 with 4k base pages) -- the size of a single pageblock.

So far the theory. Are you still running into this limitation on
upstream kernels?

--
Thanks,

David / dhildenb

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