CMA region and highmem

[Daniel Drake]

Thanks Russell for looking into this! I'm also working with a large
CMA region and had also seen a couple of OOMs which are likely the
same thing.

This will also allow for larger CMA regions to be allocated as right
now they were limited to the largest free contiguous chunk of lowmem
available and that does suffer a small amount of fragmentation at boot
time.

On Wed, Aug 20, 2014 at 8:03 AM, Marek Szyprowski
<m.szyprowski at samsung.com> wrote:
> There is no direct dependecy on low-mem. It was rather a heritage of initial
> CMA implementation, which by design worked only with lowmem (there was no
> code to allocate kernel virtual mappings from vmalloc range). The only
> requirement about CMA regions is to put them entirely either in lowmem or
> in highmem. I will prepare a patch swtiching default CMA region to highmem
> if such is available and defined region fits into it.

Just curious, to complete my understanding here...

If the CMA region is now moved into highmem, when memory is allocated
with the DMA API, such memory will now use up vmalloc space. Is that
right?

In the default 3G/1G split configuration, of the 1G, 240mb is vmalloc
space by default (and 760mb is lowmem).

So, lets say your patch moves CMA into highmem, and I build a kernel
with a 512mb CMA allocation.

Now whenever anyone uses the DMA API to allocate from CMA, it will
only be able to allocate a maximum of 240mb, later allocations will
then fail due to lack of vmalloc space. Right?

Of course, vmalloc space can be increased, but only at the expenses of
reducing lowmem.

Thanks for any clarifications!
Daniel