linux-kernel - Re: Correlation CMA size and FORCE_MAX

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Message-ID: <826cd775-14d2-12ae-2e96-cf0766aa1502@redhat.com>
Date:   Mon, 19 Sep 2022 10:38:47 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michael Nazzareno Trimarchi <michael@...rulasolutions.com>,
        Mike Rapoport <rppt@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: 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