Re: Correlation CMA size and FORCE_MAX_ZONEORDER

[David Hildenbrand]

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