Re: Correlation CMA size and FORCE_MAX_ZONEORDER

[David Hildenbrand]

On 19.09.22 11:17, Michael Nazzareno Trimarchi wrote:
> Hi David
>
> On Mon, Sep 19, 2022 at 10:38 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > 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?
>
> I can run 6-rc2 on my board. I test again but according to it, if I
> put 4M as CMA in cma=4M in boot
> parameters, the result is 32Mb of CMA. Apart of that seems that
> process lime tiny membench can not even start
> to mblock memory

The CMA alignemnt change went into v5.19. If "cma=4M" still gives you > 
4M, can you post /proc/meminfo and the early console output?


--
Thanks,

David / dhildenb